On the basis of direct measurements by UV-vis spectroscopy, the dissociation constant of bichromate was found to decrease with temperature from 160 to 400 degrees C. For fixed Cr(VI) and KOH concentrations, the molal concentration of HCrO4- initially increases with temperature but decreases again in the vicinity of water＇s critical point, where the density decreases substantially. The decrease in HCrO4- at high temperature and low density may be attributed to (K+)(CrO42-) ion pairs, to a high degree of electrostriction about CrO42-, which facilitates the reaction HCrO4- + OH- = CrO42- + H2O, and to ion activity coefficients.