Abstract: A method and apparatus for maintaining an electrically operating device at a desired temperature during burn-in testing of the device includes immersing the device in a bath of an inert liquid having a boiling point less than the desired case temperature. The device generates heat during electrical operation which is transferred to the bath by nucleate boiling of the liquid. The device temperature is monitored until it stabilizes at a temperature between the boiling point of the liquid and the desired device temperature. An inert liquid having a boiling point greater than the desired devise temperature is slowly added to the bath to modify the rate of nucleate boiling of the lower-boiling liquid, while the device temperature is simultaneously monitored, until the case temperature reaches the desired device temperature.