Abstract: A control apparatus for controlling the temperature of an environmental chamber within which a device is tested for thermal characteristics employs a pair of control elements connected in cascade. The first element produces a required air temperature in response to a desired temperature and to the temperature of the device; the second produces a temperature set point in response to the required and actual air temperatures. Possible stress on the device is avoided by limiting the range of required air temperature. Response is accelerated by a pass-through circuit, parallel to the first element, which responds to a change in the desired temperature by adding to the required air temperature of value proportional to the change.