Abstract Films of ZnS and GaAs have been deposited on air cleaved NaCl substrates by R.F. sputtering. The substrate temperatures required for epitaxial growth were determined to be 20°C or lower for ZnS, and to be 230°C for GaAs. GaAs films deposited at temperatures around 20°C are amorphous and have resistivities of about 10 5 Ω cm and activation energies of about 0.47 eV. As the substrate temperature is increased above 140°C the diffraction pattern shows that the films become initially polycrystalline and at the highest temperatures studied, about 240°C, become single crystal. Electrical measurements indicate that the resistivity and activation energy decrease to about 10 2 Ω cm and about 0.15 eV respectively. The optical energy gap of the polycrystalline films was measured to be in the range 1.29 to 1.59 eV in agreement with the value of 1.35 eV for the bulk material.