An isothermal jacket microcalorimeter, supplemented by metallographic, microhardness, and X-ray measurements has been used to study the isothermal annealing of high purity copper after room temperature tensile deformation. The amount of stored energy released during annealing has been measured as a function of deformation in the range 10.8 to 39.5 pet elongation. The data have shown the major heat effect to be associated with recrystallization and have allowed an analysis of the recrystallization kinetics and the calculation of activation energies of recrystallization.