The isotope dilution method, which is based on the isolation of a constant substoichiometric amount, can be completed by a group of radiometric techniques in which this amount is a function of the concentration to be determined. It is convenient then to plot the experimental data in such a way that a straight line is obtained. The methods which can be used in the application of this principle are discussed, mathematically treated and compared to the original procedure of RŮZICKA and STARÝ. Expressions are derived for the relative statistical error and the limit of detection.