The longitudinal optical mode shift with temperature was measured in two vertical cavity surface‐emitting laser (VCSEL) type optical resonators with different GaAs and AlAs layer structures. The measurements show distinct differences in the behavior of the cavities. From the data the thermal dependencies of the indices of refraction of GaAs and AlAs for wavelengths near 1 &mgr;m were determined to be (2.67±0.07)×10−4/°C and (1.43±0.07)×10−4/°C, respectively. ©1995 American Institute of Physics.