A theoretical study of the beta-scission reactions for some thioimidoyl radicals (R-1-N=C-center dot-S-R-2) using a recently introduced homofugality index, nu(center dot) is presented. This index, that was defined as a descriptor of the leaving group ability in homolytic substitution reactions, predicts that S-R-2 fragmentation is always favoured compared to the N-R-1 scission, in agreement with the experimental data. This result can be explained on the basis of the regional spin softness of the leaving radical group at the transition state during the homolytic fragmentation. (c) 2007 Elsevier BN. All rights reserved.