A mathematical model to describe the contamination of steel surfaces by 60Co in high-temperature water is developed. The model assumes that 60Co is incorporated into the growing oxide film on the steel, so that the contamination rate is governed by corrosion kinetics. Release to inactive coolant is controlled by solid-state diffusion and is consequently very slow. Diffusion processes for both release and activation are modeled in terms of a diffusion parameter that is characteristic of the surface oxide. The model is tested with data from recirculating and once-through loops, and good fits are obtained if either parabolic or logarithmic corrosion kinetics are assumed, although logarithmic kinetics are somewhat better.