Summary: An interpretation of part of the hydroxyl-stretching region of the infra-red absorption spectrum of biotite is proposed. A consideration of band shapes enables the interpretation to be quantified and tested against a series of natural biotites. The equation derived may be used to determine the atomic ratio Mg²⁺/(Mg²⁺+Fe²⁺) in biotites but the necessary assumptions and simplifications make it unlikely that the method will be very useful in practice.