The oceanic composition on Saturn's moon Enceladus is evaluated through calculations of thermochemical equilibria at hydrothermal and freezing settings. Conditions of rock alteration are constrained from assumptions and models for the moon's interior composition and thermal evolution, and from the composition of Enceladus' plume. Results show that an early ocean was an alkaline Na⁺-Cl⁻-HCO₃ ⁻ solution. Underlying altered rocks consisted of Mg-phyllosilicates, magnetite, Fe and Ni sulfides, and carbonates. Subsequent freezing of oceanic water caused the deposition of a NaCl hydrate, Na, K and Ca carbonates, and the formation of a salt-free ice shell. If an aqueous phase exists on today's Enceladus, it could consist of eutectic Na-Cl-HCO₃ ⁻ brine that at least locally decouples the ice shell and facilitates tidal heating. A lack of firm detection of Na and Cl at Enceladus is consistent with the accumulation of salts at the ice-rock boundary and implies the plume formation via sublimation in the ice shell.