Second-order rate constants have been measured spectrophotometrically for the nucleophilic substitution reactions of S-p-nitrophenyl thiobenzoate with various anionic nucleophiles including 6 ${alpha}$-effect nucleophiles. A good Bronsted correlation has been observed for the reactions with 7 aryloxides. However, p-chlorothiophenoxide and hydroxide ions exhibit significantly positive and negative deviations, respectively, from the Bronsted plot. The deviations are attributed to the effect of polarizability and solvation rather than a change in the reaction mechanism. The ${alpha}$-effect nuceophiles except highly basic ones demonstrate remarkably enhanced nucleophilicity. The effects of solvation and/or polarizability are proposed to be important for the cause of the ${alpha}$-effect.