Reactor irradiation of uranium monocarbide (UC) caused pronounced effects on its reactions with water and with aqueous solutions of NaOH, HCl, and H2SO4. Specimens irradiated to a burnup of 0.6 at.% or higher were essentially inert to water and to 6 M NaOH at 80°C. When the burnup was 0.06 at.% the specimens hydrolyzed, but the rates were much lower than those obtained with unirradiated specimens. The irradiation had little effect on the rates of reaction with HCl and H2SO4. When hydrolysis of irradiated UC occurred in water, 6 M NaOH, 6 M HCl, or 6 M H2SO4, the gases evolved contained less methane, less total volatile hydrocarbons and more hydrogen than the gases evolved from unirradiated UC under the same conditions. In general, with increasing burnup of the UC, the amount of hydrogen evolved increased while the amounts of methane and total carbon recovered in the gas decreased.