Czochralski silicon samples have been heated for increasing periods of time at temperatures of 395, 415, 450, 475, and 490 °C. At each stage infrared spectra of the electronic absorption of thermal donors TD2–TD9 were obtained with the samples at 4.2 K. The data for all the thermal donor centers can be presented on one universal plot, provided the concentration and time axes are simply scaled by factors that depend only on the temperature of heating. It is implied that the rate of formation of each center is limited by the same mechanism. An activation energy of 1.7±0.1 eV is determined for these processes.