The alteration of magmatic plagioclase and pyroxene phenocrysts in tephra adjacent to the conduit of a fissure fumarole from the Valley of Ten Thousand Smokes, Katmai, Alaska, was studied by electron microprobe and transmission electron microscopy. In slightly altered tephra, plagioclase compositions >An73 are not observed, the more calcic portions of the phenocrysts having been replaced by amorphous silica. With increasing proximity to the fumarole conduit, progressively more sodic parts of the phenocrysts have disappeared. Adjacent to the fumarole, plagioclase >An44 has been removed because of the increased solubility of albitic feldspar in chloride-rich solutions as a function of increasing temperature. On a submicrometer scale, a zonal sequence of replacement phases has developed at the interface between unaltered and altered feldspar. A thin leached surface layer (<500 Å), depleted in Al, Na, and Ca, but highly enriched in Si and Cl, is always present. In altered phenocrysts away from the vent, a zone ≤5000 Å of amorphous Al-bearing silica is present, locally containing secondary Al-rich smectite crystals. Closer to the fumarole, smectite is absent, and the zone of impure amorphous silica is much thinner. Extensive alteration of pyroxene phenocrysts only occurs close to the fumarole conduit, where replacement develops along fractures and parallel to (100). A narrow leached zone (200–300 Å), depleted in Mg and Ca, is present at the alteration interface, and halloysite is present locally as a secondary precipitate. The alteration of both plagioclase and pyroxene occurred when the fumarole system had cooled significantly (<300 °C) and was dominated by Si-rich, Cl-bearing aqueous fluids with pH < 2.5.