In situ sheet resistance measurements were used to study copper Films alloyed with various concentrations of cobalt during annealing at temperatures up to 1000 degreesC. The films were initially characterized by a high resistivity, several times that of pure copper. because of the presence of cobalt dissolved in copper. A large decrease in resistance, that occurs when the temperature reaches 100-150 degreesC, is likely to be the consequence of the precipitation of cobalt associated with grain growth in the copper film. In this process, the free energy change that accompanies precipitation is added to the usual driving force that causes grain growth, namely the overall decrease in interface energy. An activation energy of approximately 1 eV was found by varying the ramp rate during in situ resistance analysis. indicating the dominance of' grain boundary diffusion. Further increases in temperature led to other changes in resistance that can be attributed to precipitate dissolution, and the reaction of cobalt with a tantalum adhesion layer. The activation energy of approximately 2 eV determined for this latter reaction is characteristic of the lattice diffusion of cobalt in copper. A rather extensive discussion considers (a) the modalities of the resistance changes that may be observed during heating and cooling solid solution alloys: and (b) the use that can be made of these changes to determine such process characteristics as activation energies.