Electroreflectance (ER) spectroscopy has been carried out on the CdS/CdTe thin-film solar cells to reveal the CdS/CdTe interfacial properties, which is believed to play a crucial role in determining the photovoltaic performances. The ER feature at around 1.46 eV is to be ascribed as due to the mixed-crystal CdTe0.95S0.05 layer unintentionally formed at the CdS/CdTe interface during CdTe deposition on CdS. Comparing the ER spectrum with the quantum efficiency (QE) response of the solar cell, the indication is that the n-p junction locates itself in the mixed-crystal layer, because no pronounced response due to the CdTe layer is observed in the ER spectrum whereas it is observed in the QE spectrum.