The relationship between the signs of ${ ho}i(0)$, ${ ho}j(0)$ and ${ ho}ij$ and validity of the reactivity-selectivity principle (RSP) has been derived: RSP is valid when W = ${ ho}i(0){cdot}{ ho}j(0)/{ ho}ij$ is negative. The analysis of 100 reaction series indicated that for normal SN2 reactions involving variations of substituents in the nucleophile (X) and in the substrate (Y) RSP is valid only for a dissociative type for which ${ ho}Y(0)$ is negative, whereas for the acyl transfer reactions with rate-limiting breakdown of the tetrahedral intermediate RSP is valid in general for all substituent changes, X, Y and/or Z (substituent on the leaving group). The trends in the validity of RSP for certain types of reaction can be useful in supplementing the mechanistic criteria based on the signs of ${ ho}i(0)$, ${ ho}j(0)$ and ${ ho}ij$.