In order to study the effects of substituents on the pyridine ring, the thermodynamic acid dissociation constants of the pyridinium ion and thirty three 3- and 4- substituted pyridiniunl ions have been measured in water at 25°, together with the rate constants for the reactions of ethyl iodide with pyridine and with twenty 3- and 4- substituted pyridines in nitrobenzene at 60°. The results, which have been analysed using the Hammett equation, show that reactions on the pyridine nitrogen are very sensitive to substituent effects. Para substituents with +R characteristics are involved in direct conjugative interaction with the reactive centre. In contrast, -R substituents appear to exert inductive effects only. Meta amino and methoxy substituents show exalted substituent constants, which are a consequence of the very strong polarisation effects of the positively charged nuclear nitrogen atom. All other meta groups show little or no deviation from the Taft σ° values. The ultra-violet spectra of the thirty four pyridines in acidic and alkaline media are given, in the appendix, for the range 230 - 360 mμ.