The optical constants of thermally evaporated thin films of ZnS have been determined by measurements of transmittance at normal incidence from two films of different thickness. A single-layer model has been successfully used for the films. The results are compared with those obtained earlier for thermally evaporated ZnS films using reflectance and transmittance measurements (where a two-layer model for a ZnS film was used). The advantage of the present method over the earlier one is the readily available measurement facilities.