Correct GaAs and InP substrate temperatures are monitored with an infrared optical pyrometer. The temperature calibration for the pyrometer is carried out with the help of an emissivity change, produced by an eutectic reaction, in the Al deposited Si substrate. It is found that large apparent temperature differences exist betweenn+‐ (orp+‐) and semi‐insulating GaAs and InP substrates when constant emissivity is assumed. The temperature difference is ∼60 °C at 500 °C for InP, and 10–30 °C for GaAs. The temperature differences are found to increase with increasing substrate temperature. A model calculation is given for substrate temperature monitoring with an infrared optical pyrometer. The apparent temperature difference is attributed to the presence of optical absorption mechanisms in ann+‐ (andp+‐) substrate and its absence in a semi‐insulating substrate. The limiting temperature of congruent evaporation and the native oxide evaporation temperature for GaAs are measured with an accurately calibrated infrared optical pyrometer. They are found to be 612±10 °C and ∼585 °C, respectively.