Using vibrating optical beam, differential reflectance (DR) spectra have been obtained on GaAlAs thin films and GaAs bulk subjected to an externally applied temperature gradient. The DR spectra reveal all the critical points, namely, E0 and E0+Δ0 in GaAlAs and E0, E0+Δ0 and E1 in GaAs in the given energy range under study whereas, without the temperature gradient, DR spectra measured on homogeneous materials are structureless. The DR technique shows better sensitivity than photoreflectance on thin epilayers. Our investigation suggests that, combined with the application of a field gradient, DR can be made extremely useful for the characterization of semiconductors. © 1997 American Institute of Physics.