The hydrothermal deposition of CrO(OH) from aqueous chromium(III) chloride, nitrate, and perchlorate, and of Cr3(SO4)2(OH)5•H2O and Cr(OH)SO4•2H2O(?) from aqueous chromium(III) sulfate, has been investigated. Aqueous chromium(III) is oxidized to chromium(VI) by 0.4 M perchloric acid above 225 °C, and by molecular oxygen above 250 °C. Aqueous chromium(III) can react at 300 °C with iron or steel, cobalt, and copper to produce FeCr2O4, CoCr2O4, and Cu2Cr2O4, respectively. CrO(OH) reacts with type 316 stainless steel at 440 °C in supercritical water of density 0.7 g cm−3 to yield (Fe,Ni)-Cr2O4, which is the "cubic Cr2O3" of Laubengayer and McCune. The spinels MCr2O4 (M = Mg, Mn, Fe, Co) can be made hydrothermally at ca. 300 °C from Cr(OH)3 and M(OH)2. This information is relevant to corrosion phenomena, and the possible hydrothermal origin of chromite deposits in serpentinized rocks.