Summary: Biotites from a number of granites have been analysed for major oxides and a wide range of trace elements. Their compositions are found to be typical of biotites from granite and to show element fractionation in presumed differentiation series. Estimates are made of the fugacity of oxygen and water during the crystallization of the granites. PH2O is found to increase slightly through the differentiation series, and fO2, for a temperature of 700 °C is estimated to be between 10−18 and 10−12. Tin mineralization is connected with the igneous rocks and the relationship of tin in the micas to the rock compositions is described and the factors controlling the distribution of tin are discussed. The biotites have only up to 21% of the tin of the granites. In general Sn increases in the biotites with decreasing temperature of equilibration and with increase in the degree of albitization of the granites. High Sn in biotites seems to be an indicator of potential mineralization.