The hydrothermal decomposition of cobalt hydroxide is of importance in understanding corrosion in nuclear reactors, in the industrial production of cobaltous oxide, and potentially for thermal energy storage. The kinetics of decomposition in the presence of water vapor is poorly understood but nevertheless important in the above situations. The decomposition reaction has mainly been studied in air or inert environments. Here, we report data on the kinetics of the decomposition reaction at temperatures up to 270 degrees C in the presence of saturated water vapor. We show that CoO can be obtained as the decomposition product under a low dissolved oxygen level of <2 mg/L. The decomposition follows the Avrami Erofeev kinetics model with rate constants of 0.3 h(-1) and 0.56 h(-1) at 260 and 270 degrees C, respectively. In comparison, decomposition in N-2 and air environments showed much faster rates on the order of min(-1). Data reported here are important in the fundamental understanding of the reaction kinetics and in identifying the mechanism for the decomposition of cobalt hydroxide and other brucite-like hydroxides.