AbstractWe have carried out a kinetic study of the 4‐pyridinecarboxaldehyde plusn‐hexylamine system, at 25°C, in water‐dioxan mixtures (0–60% v/v) and in the pH range pKa+ 1.5>pH>pKa− 1.5, where pKais the pK value of the conjugate acid of the amine. The results obtained could be interpreted in terms of a rate constant for Schiff's base hydrolysis and a rate constant for the reaction between the nonprotonatedn‐hexylamine and the nonhydrated form of 4‐pyridinecarboxaldehyde. Both constants decrease sharply as the dioxan content of the solvent increases, in a manner consistent with Marshall's model [J. Phys. Chem.,74, 346 (1970)]. It is suggested that the transition state of the rate‐limiting step (carbinolamine dehydration) is highly solvated by water molecules and has a high separ