Heat is produced after interruption of the current because of relaxation of concentration gradients in electrochemical systems. This heat of relaxation is termed heat of mixing. Two methods, one computational and one analytic approximation, for computing the heat of mixing are presented. In general, the magnitude of the heat of mixing will be small in materials with transport properties sufficiently high to provide acceptable battery performance, with the possible exception of heat of mixing within the insertion particles if the particle radius is large. In insertion materials, such as lithium insertion electrodes, the entropy of reaction may vary substantially with state of charge. The entropy of reaction accounts for a reversible heat effect which may be of the same order of magnitude as the resistive heating. Measurements for the entropy of reaction as a function of state of charge in LiCoO2, LiNi0.8Co0.2O2, and LiC6 are presented.