Thin films of GaAs were grown epitaxially on single crystal sapphire substrates by a modified sputtering technique. The (0001), (0112¯) and (0221¯) substrate orientations were used at various temperatures from 150 ° to 500 °C, and the carrier concentration of about 1018/cm3 was achieved in GaAs films by the diffusion of zinc. The spectral response of the photoelectrons was measured in the incidence and transmission mode after activation by cesium and oxygen. The photoelectric yield at 2.8-eV photon energy was within an order of magnitude to that from a bulk crystal for a 0.88-μ-thick reflection photocathode. An electron diffusion length of 0.32 μ and an escape probability of 0.005 was measured for photon energies near the band edge. The photoelectric yield from transmission photocathodes was within an order of magnitude to that from the reflection photocathodes only at wavelengths longer than 0.8 μ.