Measurements of 238U capture rates (C8), relative to 239Pu fission (F9), have been carried out in 25% and 16% PuO2/UO2 fuels in the fast reactor ZEBRA using Ge(Li) detectors. A number of experimental techniques have been developed and applied, with individual accuracies of better than ±2%. The methods have made possible the elimination of any unsuspected systematic errors that might arise from the simple use of UO2 foils in mixed-oxide fuelled reactor lattices. © 1972.