Electrophoresis at high temperature was applied for studying the thermal behaviour of the membrane bound protein cytochrome b5. Approximately 95% cytochrome b5remains in the associated state after thermal transition. Thus, the transition obtained by physical methods is not due to a dissociation of the octameric cytochrome b5but to changes in chape. The described method of high temperature electrophoresis seems to be an important tool for distinguishing between dissociation and unfolding processes.