We determined the coordination environment of Zn2+ in aqueous Cl- brines at 25 degreesC and 300 degreesC using ab initio molecular dynamics simulations. The ZnCl+ and ZnCl2 complexes exist as pseudo-octahedral ZnClm(H2O)(6-m) clusters at 25 degreesC but occur as pseudo-tetrahedral ZnClm(H2O)(4-m) clusters at 300 degreesC. The ZnCl3- complex occurs as the pseudo-tetrahedral ZnCl3(H2O)(-) cluster at 25 and 300 degreesC. The tetrahedral ZnCl42- complex, however, is the dominant Zn-Cl complex at 25 degreesC, at least in highly concentrated (7.4 m) Cl- brines. The change in hydration number with temperature for the ZnCl+ and ZnCl2 complexes will complicate extrapolations of solvation energies to hydrothermal conditions using a Born-model-based equation of state.