The elimination of the residual image problem with plagued previous Mariner imaging systems allowed photometry of moderately high quality to be carried out on Mercury by Mariner 10. Relative radiance measurements are accurate to about ¿1% within a frame; absolute measurements have an uncertainty of about ¿5%. The conclusions from the photometric analysis are as follows. To a surface resolution of 20 km, Mercury is covered with a dark fine-grained oil similar to the lunar regolith. No regions having anomalous polarization or color were discovered, which might have implied large amounts of metallic Fe or exposed bare rock. Photoclinometry suggests that the center of Caloris Basin may be 9 ¿3 km below the outside rim. The heavily cratered plains have albedos of about 0.17, somewhat brighter than the lunar highlands. The albedos of the darker smooth plains and the interiors of bright fresh craters are systematically higher than their lunar counterparts. Bright craters are invariably bluer than most of the surface. One consistent interpretation of the color and albedo relationships is that the dark smooth plains resemble the low-Ti, low-Fe lunar maria and that the crust is generally low in Ti, metallic Fe, and Fe+3.