k 0 factors (composite nuclear constants) for use in the k 0 standardization of reactor neutron activation analysis were experimentally measured with great care in several laboratories. The recommended values thus obtained for the relevant gamma rays of 144 analytically interesting radionuclides are tabulated, together with evaluated values for their associated parameters such as resonance integral to thermal cross-section ratios and effective resonance energies. A classification is also given of the various activation-decay types, to which the data are strictly correlated.