An experimental study was made of alunite stability relations at 200° to 380° C and 15,000 psi total pressure. The invariant point for the assemblage alunite-muscovite-kaolinite (or pyrophyllite)-quartz in terms of total molal H 2SO 4 and K 2SO 4, respectively, is placed at 0.002 and 0.03 at 200°, 0.012 and 0.013 at 300° and 0.02 and 0.016 at 380° C. The K-feldspar-alunite-muscovite- quartz invariant point is placed by extrapolation at approximately 0.07 and 0.13 at 300° and 0.14 and 0.14 at 380° C. Conversion of the aqueous concentration data to activities of constituent species at temperature was made through the use of published data on dissociation constants for H 2SO 4 and K 2SO 4 (Quist et al., 1965, 1966). The effect of pressure on the equilibria under these moderately elevated pressure-temperature conditions is small, and Na as a component in the system, although definitive for mineral compositions, does not significantly affect the acidity of the system. The phase relations delineated in the experimental work provide information on the compositional character of hydrothermal solutions that produce strong hydrolytic (advanced argillic) type alteration. A fairly high H 2SO 4 concentration is required before alunite can be produced in equilibrium with muscovite, kaolinite or K-feldspar. Consistent with geologic observation, alunite-kaolinite (including dickite or pyrophyllite) are shown to be the most likely products of such alteration processes in either hot spring or higher temperature environments, whereas muscovite-alunite is favored by higher temperature as well as higher potassium concentrations. The phase relations have application to volcanic solfataric alteration, hypogene alunite in some silver and base metal sulfide deposits, the Bolivian tin deposits, and gold-alunite deposit.