Activated processes at surfaces such as desorption or sublimation may exhibit a thermal anomaly at the Curie point of a magnetic substrate. We propose to measure this anomaly with an applied magnetic field H, and we predict a decrease in the reaction rate proportional to Hx with x(T > Tc)=2, x(T < Tc)=1, and x(T = Tc)=2/δ1, where δ1≃1.9 is an exponent for ordinary phase transitions. In the case of a surface transition, or if the substrate is a film with two-dimensional Ising behavior, the anomaly is significantly enhanced
