This report describes a sensitive, proportional temperature controller developed by the Bureau of Mines that is useful in the temperature range from 40 deg. to 400 deg. C with an accuracy of +/- 0.1 Deg. C. A temperature controller of this kind is potentially useful for regulating temperatures in air chambers, liquid baths, furnaces, and reaction vessels, and for other applications. This instrument was developed to control the duration and temperature of the heating cycle of a charcoal-filled adsorber that is part of a special helium analyzer used by the Bureau. The controller was made from commercially available parts, and can be easily modified to provide continuous temperature control. The circuit is solid-state and employs no electromechanical devices. Over a 2-year period of use as a component of the special helium analyzer, this temperature controller performed successfully and required no maintenance.