A once-through loop containing pressurized water at 300°C and constant chemistry conditions of high pH (with LiOH) and dissolved hydrogen has been used to study the deposition and release of 60Co at heated Zircaloy surfaces. The activation of the surfaces proceeds by a mechanism involving dissolved species. The mechanism is affected little by heat flux, but a great deal by the type of oxide film formed by corrosion on the Zircaloy. To take account of the mechanism, a mathematical model has been developed. It describes first-order processes, such as adsorption/desorption or ion exchange occurring at a continuously changing surface. The model has furnished deposition and release coefficients for 60Co on thin black and thick white ZrO2 films for operation with and without heat flux.