We studied the relationship between altitude and microwave emissivity (the complement of power reflectivity) in 10 highland regions of Venus, using the Magellan data set. Above a critical altitude that ranges from 4.75 km on Maxwell Montes to 2.49 km on Sapas Mons, emissivity undergoes an abrupt decrease to values so low (<0.6) that the surface mineralogy on these mountaintops must differ from that at lower altitudes in such a way as to enhance the bulk dielectric constant of the highland surface material. This emissivity effect probably is produced not by basalt minerals but by a secondary, weathered mineral assemblage. We developed the phase diagrams descriptive of equilibrium secondary mineral assemblages on Venus, as a function of altitude and atmospheric redox state. The mineral responsible for low emissivity on mountaintops appears to be the electrical semiconductor pyrite (FeS2). Variation from one highland area to another in the altitude at which pyrite becomes the stable Fe mineral (as opposed to magnetite at lower altitudes) may be attributable to local modulation by topography of the atmospheric thermal profile. The altitudes at which the magnetite/pyrite phase boundary is encountered vary with the redox state of the atmosphere, and observed values of this altitude can be used to estimate an oxygen fugacity at plains level of ~10-21 bars. Maat Mons, a volcanic peak almost as lofty as Maxwell Montes, is the only high mountaintop on Venus that has not weathered to a low-emissivity mineral assemblage. Presumably, this means flows at the highest altitudes are negligibly weathered and the volcano is relatively young. ¿ American Geophysical Union 1992