Substituent effects of substrate and nucleophile for the reaction of substituted phenacyl tosylates with pyridines were determined conductometrically in acetonitrile. Activation parameters for these reactions were also calculated. The substituent effects in nucleophile were increased with electron-donating power of pyridines and Br${o}$nsted linear relationship was shown. Rate constant was increased by both electron-donating and electron-attracting groups in the substrate. It seems that dissociative S$_{N}$2 ("loose" transition state) mechanism is operating in the case of electron-donating substituents while associative S$_{N}$2 ("tight" transition state) mechanism is operative in the case of electron-attracting substituents.