Summary: Without direct evidence that cations favour particular sites in the octahedral layers of phlogopitebiotite micas, it is usually assumed that the cations are distributed randomly. Such a distribution, however, implies appreciable populations of cation combinations that seem electrostatically improbable. A computer program was written that rearranges a random distribution so that the improbable combinations of cations are eliminated by predetermined restrictions. Its use shows that conditions deduced from Pauling's electrostatic valence rule lead to grouping of trivalent cations around vacancies; for some chemical compositions, chains of alternate trivalent cations and vacancies extend through the structure.