Unit-cell dimensions for synthetic alunite and natroalunite formed at 450 °C are a = 6.981(1) Å, c = 17.331 (4) Å and a = 6.9786(7) Å, c = 16.696(3) Å, respectively. Synthetic alunite-natroalunite solid solutions show a linear variation in c with mole fraction Na, but this trend is obscured in natural samples by heterogeneities in Na content. The mole fraction Na in natural samples can be estimated by XNa = (17.331 − c)/0.635 with an accuracy of ± 0.10. Alunite from low-temperature environments is characterized by fine grain size (<5 μm), an anomalously large value of a, significant H2O loss during heating in a vacuum at 300–400 °C, and total H2O in excess of the stoichiometric value. In contrast, alunite from hydrothermal environments is coarser grained, has an a value identical to synthetic alunite formed at 450 °C, evolves negligible H2O when heated to 400 °C, and does not contain H2O in excess of the stoichiometric amount. The value of DD of the H2O evolved when alunite is heated at 400 °C is shifted toward local meteoric H2O relative to the bulk H2O, suggesting that the low-temperature H2O exchanges with ambient H2O vapor at room temperature.