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 sampled and analyzed >100 low-temperature (<40°C) fluids from the Samail ophiolite in Oman to test thermodynamic predictions with environmental observations. Additional simulations (e.g., fluid mixing, mineral leaching) were also conducted to account for deviations from equilibrium expectations. Type 1 circumneutral (pH 7 to 9) fluids result from fluid interactions with completely serpentinized rocks common in the shallow subsurface. Type 2 hyperalkaline (pH >11) fluids approach equilibrium with diopside, and serpentine and brucite actively forming during advanced stages of serpentinization. We also investigated fluids with pH values of 9 to 11 to test whether these fluids are indicative of intermediate stages of serpentinization or mixing between the above end-member fluids. Fluids at intermediate stages of serpentinization and fluids derived from mixing can have the same pH, but the former have considerably lower dissolved Si that can be attributed to concomitant subsurface serpentinization and mineral carbonation processes. Overall, this work demonstrates that predicted and measured compositions of serpentinization-derived fluids can be successfully reconciled using a combination of equilibrium and fluid-transport simulations. This work substantiates these calculations as useful tools in exploring serpentinization reactions in deep subsurface aquifers on Earth as well as those beyond our own planet.