A lattice-mismatched CdTe epilayer on a GaAs(100) substrate was grown by the simple method of temperature-gradient vapor deposition in the growth temperature range between 200 and 350 degrees C. From X-ray diffraction analysis, the layers grown were found to be CdTe epitaxial films. Double-crystal X-ray diffraction was used to evaluate the crystallization and the crystal quality of the CdTe films. The stoichiometry of the CdTe films was investigated by Auger electron spectroscopy. Transmission electron microscopy measurements showed that there was a lattice mismatch between the CdTe epitaxial layer and the GaAs substrate. Photoluminescence measurements at 15 K showed that a CdTe film grown on GaAs(100) in the temperature range 200-350 degrees C appeared to have an optimum crystal perfection at a substrate temperature of about 255 degrees C. Electrolyte electroreflectance spectroscopy measurements were carried out to determine the energy gap and strain in the CdTe thin films on the GaAs(100).