The stoichiometry and thermodynamic properties of Ge(IV) hydroxide complexes were generated from both solubility and potentiometric measurements. The solubility of the tetrahedral germanium oxide (GeO 2 (tetr)) was measured at temperatures from 25 to 350°C in acid to alkaline solutions at the saturated vapor pressure of the system (P sat ). Potentiometric measurements were performed on GeO 2 -KOH aqueous solutions at temperatures from 21 to 200°C and P sat using a pH solid-contact glass electrode. Results indicate that Ge(OH) 4 ° (aq) is the dominant Ge-bearing species at concentrations up to at least 0.05 m over a wide range of pH (0–8) and temperatures (20–350°C). GeO(OH) 3 − forms in significant amounts only in alkaline solutions (pH > 8–9). These results were combined with the available low-temperature solubility data on the hexagonal germanium oxide (GeO 2 (hex)) and the thermodynamic properties of GeO 2 (tetr) and GeO 2 (hex) to generate Ge(OH) 4 ° (aq) and GeO(OH) 3 − thermodynamic parameters within the framework of the revised HKF equation of state (Helgeson et al., 1981; Tanger and Helgeson, 1988). Calculations carried out using these parameters indicate that the distribution of Ge hydroxide species as a function of pH and temperature is similar to that of silicon hydroxide complexes. However, the significant differences between Ge(OH) 4 ° (aq) and Si(OH) 4 ° (aq) enthalpies of formation and heat capacities can lead to large variations with temperature of Ge/Si ratios in solutions in equilibrium with Ge-bearing silicates. For example, calculations show that the Ge/Si ratio in a fluid in equilibrium with a Ge-bearing wollastonite (Ca(Si,Ge)O 3 ) increases by an order of magnitude when temperature is raised from 25 to 500°C. This can be responsible for the high values of Ge/Si ratios measured in high temperature crustal fluids.