A systematic and convenient method has been devised for the calculation of the radiant interchange within an enclosure containing both specularly reflecting and diffusely reflecting surfaces. The new formulation makes use of simple properties of the images formed by plane mirrors, and this does away with the need to keep account of individual rays as they are reflected on the specular surfaces. With this image technique, heat-transfer calculations for a specular-diffuse enclosure are no more difficult than those for a fully diffuse enclosure. The method is illustrated by application to specific enclosure geometries.