This study evaluated the reaction kinetics of hydrothermal synthesis for the nickel nitrate to nickel oxide reaction that occurs in a wide range of temperatures and pressures around the critical point of water. The effects of mass transfer on the reaction rate were evaluated using a two-flow mixing system from which the relation between the mass transfer rate and Re number and the kinetics under reaction control conditions was obtained. The hydroxyl ion concentration, which varies greatly around the critical point, strongly affects the reaction kinetics. Results suggest that the hydroxyl ion is a reactant. With consideration of that fact, the intrinsic reaction constant was evaluated. Near the critical point, the properties of water change greatly with even slight changes of temperature and pressure. Strong solvent effects on the intrinsic kinetic constant were observed. An acceleration of intrinsic kinetics above the critical temperature or with reduced pressure in a supercritical condition is attributable to a decrease in the dielectric constant. This dielectric constant effect on the intrinsic rate constant from gas-like to liquid-like atmospheres can be explained well using the Kirkwood equation, which suggests a missing link between Eyring's gas-phase absolute kinetics theory and the Kirkwood liquid phase solvent effect theory.