Abstract The use of planar reflective surfaces can substantially improve the performance of both active and passive solar collectors. In this paper the results of theoretical calculations and experimental tests are presented on the use of different types of flat reflective surfaces to increase the collection of solar energy by flat collectors. Specular, diffuse, and combination specular/diffuse reflective surfaces are discussed. This present work differs from that of other investigators principally in that an attempt has been made to describe the reflective properties of surfaces in more generalized terms than simple specular or simple diffuse. Most real surfaces possess a combination of specular- and diffuse-like reflectivities. The reflectivity properties of a given surface can be measured in the laboratory as a function of incident and reflected angles, and these measured reflective properties can be used in the computer model to predict the increase in collector performance with such a reflector. Thus outdoor tests of a given reflector can be avoided if desired, and yet it is possible to make an estimate of the reflector's contribution to the collector's overall performance. Theoretical calculations of collector energy inputs were done for several distinct types of reflecting surfaces. These calculations based on indoor laboratory measurements of the reflective properties of the surfaces, were compared with experimental results obtained from an outdoor simulation apparatus. Predictions of system performance were made for various collector/reflector configurations, and compared with the performance of an optimally oriented collector without a reflector.