The compositions of the cores and margins of the plagioclase phenocrysts in glassy acid rocks have been used to derive equilibration temperatures with their enclosing liquids using the Kudo-Weill geothermometer. The equilibration temperatures inferred from the compositions of the feldspar margins at 0.5 kilobars water pressure, or less, agree well with the temperatures obtained from the coexisting iron-titanium oxide microphenocrysts. These low water pressures are supported by the water fugacities deduced from the coexisting biotitemagnetite-sanidine assemblages. Acid magma represented by these rocks has a crystallization interval in excess of 100° C and liquidus temperatures above 900° C.