A fast routine method for the determination of tin in rocks is discussed. The method is based on coprecipitation of tetravalent tin with ferric hydroxide, followed by a short irradiation in a high thermal neutron flux, extraction in toluene from 1∶1 sulphuric acid which is 5N in potassium iodide, and counting of123mSn (T=40 m) or125mSn (T=9.7 m) with a well-type NaI detector. In the present work125mSn was used. The lower limit of determination is governed by the blank of the reagents. For a sample of at least one gram it is ≌ 0.2 μg Sn·g−1.