The effect of temperature on the compatibility of a series of alkyl ether sulfonates AES (AE;) with synthetic brine having high salinity and hardness (brine B1) has been investigated. Three surfactants were used in this study, iC17EOXS(C17E0x (x = 7 and 10) and nC17E010S(1C17E010) along with their analogous nonionic surfactants iC17EOX (X = 7 and 10) and nC17E010. In addition, nine nonionic alkyl ether AE surfactants (100% active) were also included in this investigation, namely iCgEOX (x = 6, 10, 14), iC10EOX (x = 10, 13, 18) and iC13EOx (x = 10, 13, 18). Depending on their molecular structures, AES (AE;) exhibited concentration-dependent clear point and cloud point behavior in synthetic brine. The cloud point was associated to the non sulfonated component (AE;) whereas the clear point was attributed to the sulfonated one (AES;). Interestingly, the cloud point of the nonionic component was significantly increased in the presence of the anionic component compared to the cloud point of the same AE surfactant alone (100% active). In addition, branching has a strong effect on the clear point of the anionic AES component as reflected by a clear point below room temperature in iC17E010S(iC17E010) micelles compared to a clear point above the boiling point of water in nC17E010S(iC17E010 micelles. Likewise, increasing the degree of ethoxylation by three ethylene oxide (EO) units results in a significant decrease in the clear point from above 100 °C in iC17EOS(iC17E07) to below room temperature in iC17E010S(iCj7E010). These observations were attributed to the effect of surfactant structure on its solubilization in water. The addition of an extrinsic AE (100% active) to iC17E02S(iC17E07), iC17E010S(iC17E010), and nC17E07S(nC17E07) resulted in a significant enhancement of their compatibility with brine. Clear and stable surfactant mixtures were observed in a wide range of mixed micelle composition and temperature. Particularly, our results showed that i) the clear point of the anionic component AES is significantly decreased from above the boiling point of water to below room temperature, ii) the cloud point in mixed micelles is strongly influenced by the addition of the extrinsic AE surfactant, and iii) both above effects are correlated The variation of the cloud point in iC17E07S(C17E0YiC9E014 system with mixed micelle composition revealed three distinct modes of variation i) region I: an increase from 68 °C (cloud point in C9E014 single micelles) to 73 °C (Xan= 0. 220), ii) region II: a decrease from 73 °C (Xan = 0.220) to 67°C (Xan = 0.379), and iii) region III: cloud point below room temperature (Xan = 0.492 - 0.836). This behavior suggest that dehydration of EO groups via intramolecular ion (SO3 )-dipole (0-CH2) attractive interactions (factor promoting cloud point depression) may also compete with electrostatic repulsion between mixed micelles (factor promoting cloud point increase). Due to the presence of an intrinsic nonionic AEi component in AES(AE) surfactant, we described the nonionic mixture iC17E07/iC9E014 by an equivalent single nonionic alkyl ether surfactant characterized by its lipophilic to hydrophilic ratio Rih = . Interestingly, a clear correlation between the variation of the ratio Rih with Xan and the variation of the cloud point with Xan was observed as reflected by three distinct modes of variation occurring within a range of mixed micelle compositions similar to that observed with the corresponding variation of the cloud point with Xan This suggest that the ratio Rin plays a dominant role in the variation of the cloud point with Xan in iC17EO S(iC17EO)/iC9E014 system, along with other factors such as mixed micelle charge and ion-dipole interactions. The effect of mixed micelle composition on mixed micellization properties (CMC surface area occupied per surfactant molecule a.) in iC17E07S(iC17E07)/C9E014/brine B1 showed an evident correlation between the variation of the CMC and a, in one hand and the variation of the cloud point with Xan in the other hand. Thus, mixed micellar structure (size, shape, packing of the hydrophilic groups at the interface,..) appears to play also a significant role in the variation of the cloud point with Xan. The effect of salinity on the cloud point of iC17E07/iCEO14 mixture in iC17E07S(C17E07)/iC,E014 system revealed that at any given Xan, the cloud point monotonically decreases with salinity. We attributed this cloud point depression to the swamping of original charge distribution and shielding of repulsion between micelles with the addition of electrolytes. However, the variation in the cloud point with mixed micelle composition is independent of the total salinity and hardness and is only determined by the mixed micelle composition as reflected by the fact that the optimum cloud point value occurs at Xan = 0.220, whatever the brine composition and concentration. The effect of hardness (Ca2+ and Mg2+) and NaCl on the cloud point of iC1E07)/iC,E014 and iC17E07/i<10E014 mixtures in iCj7EO S(iC1E07)/iC9E014 and iC17EO S(iC17E07)/iC1E014 mixed micelles was also investigated in brine 1 (BI), softened brine B1 (removal of Ca** and Mg* from brine B1), and desalted brine B1 (removal of NaCl from brine B1). The trend in the variation of the cloud point verşus total salinity is again independent of the salinity and increases in the order desalted brine > softened brine > synthetic brine i,e, in the order of decreasing electrolyte (Na*, Cat, Mg) concentration: 2.50 M (B1)> 2.1 M (softened water) > desalted water (0.41 M). This was attributed, to some extent, to the fact that the nature of charge distribution at the mixed micelle-water interface is mainly determined by the binding counterions Na*, Mg2+, and Ca2. Finally, although iC1E02S(iC17E07) is not compatible with Lekhweir reservoir conditions (brine B1 and T = 70 °C), clear and stable iC17E07S(iC17E07)/iC9E014 surfactant mixtures were formulated for oil-in-water microemulsion flood in Lekhweir reservoir (oil solubilization parameter 5 = 50 at 0.025 wt% total surfactant concentration and IFT = 3.4 x 102mNm').