Abstract: Members of an alunite-natroalunite solid solution series occur in intimate association with hydrated halloysite in deposits within caves of the Guadalupe Mountains, namely Carlsbad Cavern and Lechuguilla Cave. The alunite and natroalunite crystals consist of cube-like rhombs; crystal diameters range from 0.5 to 8 µm. This mineral association is found in sediments within bedrock pockets, solution cavity fills, floor deposits and wall residues. Sulfur stable isotope values (δ34S, CTD) for cave alunite and natroalunite are negative [+0.1 to −28.9 per mill (‰); n = 12 and mean = 16.8‰] and are comparable to the cave gypsum and native sulfur values reported by other investigators. The association of alunite/natroalunite with hydrated halloysite in these cave deposits suggests that the cave-forming waters contained significant concentrations of sulfuric acid. Formation of these minerals is related to the excavation of the carbonate rocks that formed Carlsbad Cavern, Lechuguilla Cave and other caves of the Guadalupe Mountains. The sulfuric acid-bearing waters, when exposed to clay-rich sediments, converted clay minerals and quartz to alunite/natroalunite and hydrated halloysite.