Optical constants of vacuum evaporated films of zinc sulphide and germanium of some microns thickness were determined by three different methods in the region 2000-10 000 cm−1(5-1μ). The refractive indices of both substances proved to be definitely higher than for the corresponding bulk material. If the films were evaporated with pure nitrogen as residual gas instead of air, the refractive index decreased considerably and reached, for ZnS, approximately the bulk value, whereas for Ge it still remained somewhat higher. The variations in the modes of determination seem to exclude the observed anomalies being due to complexity of the film structure.