The determination of ratios of spectrum-averaged cross sections from isotopic concentrations is described along with their application to verification of analytical reactor design tools. The determination is made using specially developed least-squares computer programs which fit the mathematical transmutation relationships to isotopic concentrations measured in irradiation experiments. These data are utilized as a basis for evaluating the accuracy of calculational methods used to predict burnup behavior of nuclear fuels. Data obtained from irradiation experiments using plutonium-aluminum alloy fuel are given to illustrate the techniques and demonstrate how these data are used to verify calculational methods.