We describe the use of a simple voltage stabilizer that controls filament temperature, Tp in the ion source of a thermal ionization mass spectrometer (TIMS). The filament voltage, Vp 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^.is directly proportional to T^-in a wide range of filament temperature. The T(. value is solely controlled by the reference voltage, V^, which can be manually selected from a voltage divider or by means of a computer. The temperature controller described here is succes-fully applied for analysis of potassium concentration by the isotope dilution method.