A method has been developed to determine quantitatively the optical parameters n and k of isotropic absorbing materials using the reflected light microscope. The method makes use of nonperpendicularly incident linearly polarized light which undergoes upon reflection different degrees of reflectance for the p- and s-components. Rather than determining the absolute intensities, IRp and IRs, the ratios of these two quantities were measured. This reduces the influence of nonevaluatable sources of error such as surface imperfections, color aberrations in the instrument and detector, and variations in the size of the measuring site at different angles of incidence.The IRp/IRs ratios are related by Fresnel's reflection laws to n and k. A graphical solution of these equations is preferred over a numerical one since, by expressing the ratios as curves in (n, k) space, the intersection of these curves will determine the probable error and the necessity for more data. When the (n, k) curves intersect at small angles, axial ratios of the reflected elliptically polarized light have been used as an additional source of information. Tables have been prepared for the ratios of IRp/IRs as a function of n, k and the angle of incidence phi i and for the elliptical axial ratios as a function of IRp, IRs, their phase difference and n, k, and phi i.The instrument developed for this purpose is a reflection microscope fitted with a photomultiplier tube detector and mounted on the stationary arm of an optical goniometer. The light source and a universal stage objective (32 X ) as illuminator are mounted on the movable arm. Diaphragms and slits limit the half-cone angle of incidence to 1 degrees ; however, a correction factor may be required for large angles of incidence (phi i > or = 70 degrees ).It was observed that photomultiplier tubes are very sensitive to the polarization direction of light, a condition to which little attention has been paid in microreflectance measurements.The data for n and k corrected for polarization sensitivity, half-cone angle of incidence and other systematic errors are considered to be the best available for ore and opaque minerals. Relative errors range between 0.5 and 1.0 percent for n and between 1 and 2 percent for k. A critical review of the methods used by other investigators in determining n and k with the microscope indicates that their use results in a considerably larger error. The nature of ore minerals and their intergrowths renders the standard methods of determining n and k used for metals not applicable to natural minerals.Dispersion of the optical parameters for ore minerals is at least an order of magnitude larger than that for transparent minerals. Numerical data for this dispersion will be the key for explaining the optical properties of opaque minerals in reflected light microscopy.The variations of n and k for a single mineral species are larger than might be expected and constitute, where such could be confirmed (tennantite), a magnification of variations in chemical composition or lattice parameters. This implies that the availability of a method for the determination of n and k opens a new field of research in ore and opaque rock-forming minerals.