Polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films (<100 nm thickness) have been spin-cast from chloroform solution onto cleaved mica surfaces (roughness within 0.2 nm). An algorithm for calculating the film thicknesses based on the relative intensities of the C 1s peak of the films and the Si 2s peak of the mica from angle-resolved X-ray photoelectron spectroscopy (XPS) is presented. The film thickness changes as a function of casting conditions. Data from this approach are comparable with thickness measured by an atomic force microscopy (AFM) tip-scratch method in the range 1.5-5.5 nm. Thicknesses of the films are shown to increase linearly with concentration of cast solutions.