A simple, sensitive and reproducible method is described for the determination of tin in silicate rocks at the ppm level. By applying a selective decomposition it seems possible to differentiate between tin present in the silicate lattice, in readily-accessible cassiterite (SnO2) and in cassiterite enclosed in the silicate. The final determination is made by extraction and photometry with phenylfluorone. Results for total tin agree well with those obtained by X-ray fluorescence