Determination of very low-level activity in samples using -spectrometry is possible due to the low background. In order to determine the chemical yield in the separation of the element of interest, a spike must be added to the sample. In many practical cases there is no adequate spike or it is not available in the laboratory. In these cases a spike of the radionuclide already pesent in the sample can be used as the so-called inner standard method. In order to achieve the best results the optimal quantity and the spike to choose must be determined to reduce the uncertainties associated with the activities. The present work is a systematic study of these problems. These considerations were used to the determination of thorium in environmental samples.