Results obtained from a study on junction properties of n-CdS/p-InP heterodiodes which were prepared as solar cells by the close-spaced technique are described. The junction properties were examined by means of injection electroluminescence. It has been shown that these diodes have very low interface state density at least in deep levels even for an as-grown sample from the fact that only an electroluminescence due to the band-to-band transitions was observed at room temperature. At liquid nitrogen temperature, however, luminescence spectra originated from both band-to-band and free-to-bound transitions were observed. The energy difference between the two spectral peaks was found to be 67 meV, which corresponds to the activation energy of typical acceptors in InP. The internal electroluminescent quantum efficiency was 12.8% at 77 K.