A newinsitutechnique has been developed to measure the growth surface temperature of a substrate, and to calibrate the temperature of substrate holders. The technique has been demonstrated to be accurate to better than 5 °C and was verified for calibrating substrate temperatures between 160–220 °C. Condensation of an incident Te2flux occurred on CdTe surfaces when the flux of the incident Te2dimers exceeded the flux desorbing from the surface. The surface temperature at the onset of condensation was calculated by equating the expressions for the incident Te2flux and the desorbing Te2flux. Experimentally, the onset of condensation was determined by a change in the reflection high‐energy electron diffraction pattern of the substrate.