Variations in the H+ and O2- content of minerals have been virtually ignored since the advent of the electron microprobe in the mid-1960s. Such studies do not serve as a basis for consideration of the variation of other major constituents such as Fe3+, H+, and O2-. We report results of a comprehensive study of all major element compositional variation in biotite using samples from a suite of metapelites ranging from lower garnet to sillimanite + K-feldspar grades. The data show that Fe3+ remains constant within analytical error while percentages by weight of H2O and O decrease and increase, respectively, with increasing metamorphic grade. H+ and O2- contents of biotite appear to be highly sensitive to temperature and pressure. Samples from low metamorphic grades contain bulk H+ contents >4H+/formula unit. Excess H+ probably occupies "vacancies" in the interlayer sites as either H2O or H3O. These results will have an impact on geoscience disciplines that make assumptions about stoichiometry of hydrous phases. For example, the temperature range of reactions involving the breakdown of hydrous phases with more or less H+ than predicted will change, in some cases dramatically, and any calculations involving water activities must be modified to consider H+ variation. Partitioning of H+ among coexisting hydrous phases (δD studies) may be affected by both amount of H+ and its location and bonding (interlayer H2O or H3O vs. OH-). Ar diffusivities calculated for 40Ar/39Ar work will be influenced by the H+ occupancy in the interlayer sites. Cation substitution models for H+-bearing phases need to consider exchanges involving both H+ and O2-.