The dc conductivities and dielectric breakdown strengths of zinc sulfide thin films deposited by various techniques were investigated. The void volume in a film was a function of the deposition method and conditions, and the breakdown strength was inversely proportional to the void volume. The shape of the current-voltage curve depended on the deposition method. Films deposited by thermal evaporation and low temperature metallorganic chemical vapor deposition had void volumes in the 3 to 5% range, breakdown strengths ca. 2 MeV/cm, and exhibited little bulk conduction before breakdown. The current-voltage data were consistent with current-controlled negative differential resistance and filamentary conduction. Breakdown was due to Fowler-Nordheim tunneling of charge from the contact into the dielectric layer at all temperatures. Layers formed by sputtering a ZnS target in argon or argon/oxygen had void fractions ranging from 0.5 to 5%. Layers with low void volumes had breakdown strengths > 3 MeV/cm. The current-voltage data for sputtered films indicated initial Poole-Frenkel conduction followed by a continual increase in current with applied bias at room temperature. At 77 K breakdown occurred as a result of tunneling.