Angle of Incidence Reflectance Spectroscopy (AIRS) of specular surfaces of condensed materials (solids and liquids) provides detailed information on the materials' intrinsic properties. By measuring the reflectance at one or more large angles of incidence for radiation, polarized either parallel (Rp) or perpendicular (Rs) to the plane of incidence, one gets the information necessary to solve the Fresnel relationships for the components of the complex dielectric function at the incident radiation wavelength. Experimental difficulties with absolute reflectance measurements make it desirable to measure the ratios RP /RP at a minimum of two angles of incidence. An algorithm for AIRS has been devised for use with optically isotropic materials, which requires Rp/Rs values for two angles of incidence (preferably which include the pseudo-Brewster angle OB). When a thin (d1 /A <0.05) transparent layer coats the substrate, one can solve directly for two of the four parameters (substrate indices n, k; layer index n1; and layer thickness di) given the other two. An example is given for an InGaAsP specimen where k is known, or can be reasonably estimated, as well as the value of n1. Alternatively, when n1 and d1 are known, the substrate indices can be established. The experimental apparatus is essentially the same as that for pseudo-Brewster angle mea-surements. However, because AIRS does not require the direct determination of 'PB and the ratio at that angle, the gathering of spectral data is greatly simplified.