Abstract The reaction of pyridine bases with aliphatic monocarboxylic acids has been studied by means of the partition method. Partition was carried out at 25 °C between benzene and 0.10 mol dm−3 (Na, H) ClO4 aqueous solution, the total concentration of pyridine base and that of carboxylic acid being less than 2×10−2 mol dm−3 and 1.0 mol dm−3, respectively. Both (1:1) and (1:2) complexes (base to acid ratio) are formed in benzene, (1:3) complex being additionally formed only for 2,4,6-trimethylpyridine. A linear free energy relationship is observed between the formation of the (1:1) and (1:2) complexes and the protonation in water of the corresponding bases except for sterically crowded 2,6-dimethyl derivatives of pyridine. The structure of the (1:2) and (1:3) complexes is discussed.