The hydrolysis of uranium oxycarbide (UCO) was studied by injecting water vapor into a fuel element containing a known number of high‐temperature, gas‐cooled reactor (HTGR) coated particles with exposed fuel kernels. The experiments were conducted in the High Flux Isotope Reactor (HFIR) at temperatures between 700° and 1000°C, a system pressure of 200 kPa, and partial pressures of water vapor between 21 and 199 Pa. The general sequential response of the exposed fuel kernels to water vapor addition consisted of (1) a rapid release of stored fission gas with a concomitant increase in the steady‐state release and (2) a period of constant steady‐state release. Upon cessation of the addition of water vapor, a decline in the release to prehydrolysis values generally occurred. The release of stored fission gas was dependent on the square of the partial pressure of water vapor. The ratio of the constant steady‐state release to the prehydrolysis value was independent of the partial pressure of water vapor. The time constant for the decline in the release was the same in all of the hydro