Analytic expressions for the reflectance of an isotropic thin film with flat, parallel sides on a semiinfinite substrate are developed for the thin film limit. Proper expressions are given for the limiting cases of ideal dielectric substrates (kb = 0) and metallic substrates (kb ≫ 1). The interesting case of the intermediate substrate (kb ≈ 1) is treated for the first time. The reflectance spectra in the limiting cases are written as linear combinations of the transverse optical and longitudinal optical energy-loss functions of the thin film. Spectra of films on an intermediate substrate with constant extinction coefficient will have distorted line shapes as a result of contributions from the real part of the film's dielectric function. For a substrate with an absorption resonance, additional bands from the substrate will appear, one of which shifts in frequency with incident angle, even in a spectrum normalized to the reflectance of the bare substrate. © 1989 American Chemical Society.