A method for determining both the thickness and the average stoichiometry of thin films is presented. The method is based on implanting radioactive α-sources in the substrate prior to layer growth and measuring the energy loss of the α-particles as they traverse the layer. Information about the stoichiometry is obtained through the comparison of the energy loss of α-particles of different initial energies. Experimental examples for the utilization of this method are presented, in which Sb was grown on Si substrates, GaAs, InAs and AlAs on GaAs and YBCO on YSZ. The experimental precision which can be expected using the method is discussed, together with specific scenarios in which it could be advantageously applied.