Abstract: A method and apparatus for the continuous determination of gas-carried alpha activity caused by the decay of thorium, uranium, plutonium and their decay products. In this method, the gas-carried alpha activity from the decay chains of radon 220 and/or radon 222 is subtracted from the total gas-carried alpha activity. The alpha activity from radon 220 and/or radon 222 is measured after an absorber which only transmits the alpha rays of polonium 212 or of polonium 212 and polonium 214.