A simple and approximate model of the dynamics of the title reactions based on a Morse curve description of bond cleavage and formation is presented. The quantitative rate data concerning both anion radical cleavage and formation that are presently available are correctly reproduced by the model. It also allows one to rationalize the general trends revealed by a number of semiquantitative experimental investigations of the two reactions. The model may therefore provide guidelines for predicting, on the basis of the estimation of thermodynamic characteristics of substrates and products, the overall efficiency and selectivity of S(RN)1 reactions since the cleavage of the substrate anion radical and the formation of the product anion radical are two essential steps of the chain propagation process.