AbstractThe effective resonance integral of thorium oxide rods has been determined as a function of their surface-to-mass ratio. The range ofS/Mvalues covered is 0.15 - 0.65 cm2/g. An experimental technique based on the comparison of activities obtained in thermal and slowing-down neutron fluxes was employed. The shape of the resonance neutron spectrum was determined from measurements with a fast chopper and from calculations, permitting deduction of a correction factor which relates the experimental values to the ideal 1/Ecase. The results are summarized by the following expression:The main contribution to the margin of error arises from the uncertainties in the 4% spectral correction applied, in the 1.5 b“l/v”part deducted and in the 1510binfinite-dilution integral of gold, used as a standard. In order to compare the consistency of Dresner's first equivalence theorem and Nordheim's numerical calculations relative to our results, the resonance integral values for thorium metal rods obtained previously by Hellstrand and Weitman have been recalculated, using recent cross section and spectrum data. The new formula isIt differs from the old one mainly because of the proved non-l/vbehaviour of the thorium cross section below the first resonance.