Automatic transmission clutch failures and rapid oxidation of the transmission fluid have sometimes been attributed to high temperatures in the clutch pack. For the purpose of estimating these temperatures, an analysis was made of the clutch system, and a solution was developed from the partial differential equations of heat conduction with appropriate boundary conditions. The solution thus obtained was verified experimentally.There were several important conclusions to this study. The maximum temperature rise occurs somewhat earlier than the end of the lockup time. The interface temperature rise increases as the lockup time decreases for a fixed amount of energy input. The ratio of heat flux entering the reaction plate to that entering the composite plate is not constant, as has been assumed by others. The heat absorbed by the fluid outside the clutch pack is negligible during the short lockup time. Temperature rises for some typical cases are computed.