Pyrometric interferometry (PI) has recently been demonstrated for simultaneous real time wafer temperature and thickness measurement during the molecular beam epitaxy process. Both parameters of the thin film layer can be determined from the changing interference conditions in the layer. We used a reflection assisted version of PI to follow the thermal history of the wafer under different conditions and were able to resolve temperature to less than 1 °C. For thickness measurements, a parabolic fitting algorithm was used to accurately determine the endpoints of the GaAs/AlAs quarter wave stacks. Compared to other noncontact methods this technique can be used for very thick layers and is unaffected by the layer absorption and optical effects.