Pyridin-2-amine 1-oxides react with acetic anhydride in dioxan and with p-nitrophenyl acetate in water by rate-determining O-acetylation followed by rapid intramolecular rearrangement to give the amide. The rearrangement is so favoured that no hydrolysis of the intermediate was detected in the aqueous solvent. This behaviour in aprotic solvents contrasts with that of pyridin-2-amine, previously shown to undergo acetylation directly at the amino group. Acetylation of pyridin-4-amine 1-oxide, with acetic anhydride in acetone, occurs by way of an analogous O-acetyl intermediate which reacts slowly with a second molecule of amine 1-oxide. Pyridin-Camine reacts in the same manner at a surprisingly similar rate. In water, both compounds catalyse the hydrolysis of p-nitro-phenyl acetate, with the amine 1-oxide being very much less efficient.