We describe a reproducible and precise temperature control system for the molecular beam epitaxial growth of AlGaAs at substrate temperatures in the 700 °C range. Coating of the optical pyrometer window was observed to dramatically affect the pryometer readings making it an unreliable temperature measurement technique. By observing pyrometer interference oscillations during both GaAs and AlGaAs epitaxy, the Ga desorption rate during AlGaAs growth was estimated. The known temperature dependence of the Ga desorption rate allowed the substrate temperature to be calculated, and corrected, when necessary, during the growth. This method allowed the absolute AlGaAs substrate temperature to be controlled within ±3 °C over a six month production cycle.