An experimental method of determining “initial conversion ratios,” the number of Pu239 atoms produced for each U235 atom destroyed, is described. The measurements are made relative to the conversion ratio that would be obtained for thin uranium in a thermal flux. The precision is about ± %. The relative neptunium and fission product activities induced in a representative cross section of the fuel material (a thin foil of natural uranium) and in a foil in a thermal flux are compared. The neptunium is counted by a coincidence method which suppresses the counting rate due to fission products and natural background 10 times relative to the neptunium counting rate.