Abstract We describe the use of a simple voltage stabilizer that controls the filament temperature (T f ) in the ion source of a thermal ionization mass spectrometer. The filament voltage (V f ) is measured by means of a separate pair of wires connected inside of the ion source in parallel to the wires supplying power. It has been demonstrated that V f is directly proportional to T f in a wide range of filament temperature. The T f value is solely controlled by the reference voltage (V r ) that can be manually selected from a voltage divider or by means of a computer. Digital signals from the computer in the form of a series of pulses are transmitted opto‐electronically and subsequently converted to analog signals. The temperature controller described here was successfully applied for analysis of potassium concentration by the isotope dilution method. Copyright © 2001 John Wiley & Sons, Ltd.