Biotite, white mica, and chlorite record petrological processes and pressure–temperature conditions. Unfortunately, their crystal chemistry is complicated and thermodynamic modelling of their phase relations is hindered by this complexity. This article reviews the main atomic substitutions in metapelitic biotite, white mica, and chlorite and their relevance to mixing models. A database of chemical analyses, including major cation content, iron speciation, and H2O content of metapelitic biotite, white mica, and chlorite, is used together with a series of analyses of white mica and chlorite of metamorphic origin and a compilation of crystal structures of biotite. Emphasis is first placed on the calculation of formula units and the effect of anionic bases, which may vary considerably from their ideal values in biotite and chlorite. The analytical precision of volatile components remains of first-order importance for establishing formula units, with most phyllosilicates showing significant excess or depletion of structural water if measurements are taken at face value. When analysis is conducted by electron microprobe, as is frequent in mineralogy and metamorphic petrology, normalization to a hypothetical anionic base appears optimal in biotite if deprotonation after Ti incorporation is accounted for. For chlorite and white mica, ideal anionic bases may be used in the absence of alternatives. As for iron speciation, assuming a fixed anionic basis has consequences for calculated octahedral sums and, in turn, the extent of the di-trioctahedral substitution. Despite these uncertainties, the di-trioctahedral substitution is important in all three phyllosilicate types. Other significant exchange vectors include K–Na exchange in mica and Tschermak, Fe2+–Mg, and Al–Fe3+ exchanges in all three phyllosilicates. Low interlayer contents in mica are best modelled as solid solutions towards minerals with cation-free interlayer spaces, such as pyrophyllite for white mica and talc for biotite. Elemental ratios are useful for the graphical representation of electron microprobe measurements as they do not require normalization to anionic bases. Cation site distribution schemes are important for thermodynamic modelling. This study reviews classical approaches, highlighting possible improvements and offering a simplified allocation scheme for tetrahedral Si–Al ordering applicable prior to thermodynamic modelling. A set of independent compositional end-members is proposed for all three phyllosilicates, and the need for systematic studies of the presence of tetrahedral Fe3+ in metamorphic phyllosilicates is highlighted. The database is provided in the Supplement with spreadsheets for formula unit calculation, cation site distribution schemes, and visualization of compositional data.