Thermodynamic calculations provide valuable insights into the reactions that drive the profound fluid transformations during serpentinization, where surface fluids are transformed into some of the most reduced and alkaline fluids on Earth. However, environmental observations usually deviate from thermodynamic predictions, especially those occurring at low temperatures where equilibrium is slowly reached. In this work, we analyzed 138 low-temperature (40°C) fluids from the Samail ophiolite in Oman to test thermodynamic predictions with environmental observations. Four fluid types were identified through this work. (i) Type 1 circumneutral (pH 7–9) fluids result from fluid interactions with serpentinized rocks common in the shallow subsurface. (ii) Fluids with pH ranging from 9 to 11 and low Si concentrations are products of intermediate stages of serpentinization. (iii) Type 2 hyperalkaline (pH > 11) fluids approach equilibrium with diopside, and with serpentine and brucite actively forming during advanced stages of serpentinization. Lastly, (iv) most fluids sampled in this work deviate from predicted equilibrium compositions and depict various degrees of mixing between Type 1 and 2 fluids. Mixed fluids fall within the same pH range but have considerably higher dissolved Si than intermediate-type fluids. Hyperalkaline fluids exhibit variable degrees of mixing despite maintaining pH > 11, implying strong buffering capacity of serpentinization-generated fluids. Overall, this work demonstrates that predicted and measured compositions of serpentinization-derived fluids can be reconciled using a combination of equilibrium and fluid-transport simulations. This work substantiates these calculations as useful tools in exploring serpentinization reactions in continents and perhaps in other low-temperature environments on Earth and beyond.