The true and apparent temperature of samples during the deposition of III-V layers by molecular beam epitaxy changes as a result of the variation in spectral emissivity ϵ with layer thickness. Taking into account the infrared optical properties of these materials, we modelized the variations of the true sample temperature and the apparent temperature (as determined by pyrometric measurement) during the growth. We limited our study to deposits involving at least one absorbing material (at the pyrometer wavelength), for example GaSb, InAs or InSb. We showed that our simple model can agree reasonably with experiments in the 400−500 °C temperature range.