The work presented in this thesis is divided into three main parts. In the first two parts we investigate nanoparticles of Sn-doped a-Fe2O3 (Sn concentration: 0%, 3%, 5%, 8%, 10%, and 13%) and Sn-doped Fe304 (Sn concentration: 0%, 1.5%, 3%, and 5%) prepared by the reflux method. In the third part, we study the influence of Sn- concentration on the milling-induced transformation of Sn-doped Fe3O4 to Sndoped a-Fe2O3. The main techniques used in the study are X-ray diffraction (XRD), transmission electron microscopy (TEM) and Mössbauer spectroscopy. For the Sn-doped a-Fe2O3 system, it was found that the introduction of Sn in the corundum-related structure decreases the crystallite size by -50% at Snconcentration of 5% but stays almost the same for higher concentrations. When Snconcentration increased up to 5%, the lattice parameter a increased with almost constant c lattice parameter, whereas the c lattice parameter increases faster at higher Sn-concentrations. The shape of the nanoparticles changed with Sn-concentration from almost spherical at 0% to oblate at 3% to rod-like (100-200 nm in length) at 5% and returning to the spherical shape at higher Sn-concentration. The Mössbauer isomer shift values increased whereas the effective hyperfine magnetic fields decreased with increasing Sn-concentration. The presence of Sn** suppressed the Morin transition For the Sn-doped Fe3O4 system, it was found that both the crystallite size and the lattice parameter (a) increase with increasing the Sn concentration in the spinelrelated structure. For all samples the particle size was found to be in the range of 1050 nm. The Mössbauer isomer shift values were larger for the octahedral (B) sites than the pure sample and their effective hyperfine magnetic fields decreased with increasing Sn-concentration. This is indicative that Sn** ion substitute Fe ones octahedral (B) sites. . The milling of the Sn-doped Fe3O4 samples (Sn-concentrations of 1.5%. 5 %, 3% and 5%) in air for different times transformed them to Sn-doped a-Fe2O3 nanoparticles via an intermediate y-Fe2O3 related phase. The time needed for a complete transformation was found to decrease as Sn-concentration increases from 71h for 1.5% Sn-doped Fe3O4 sample to 33 h for the 5% Sn-doped Fe3O4sample.