3-Hydroxyisothiazole exists as the lactim in nonpolar solvents, the lactam form predominating in aqueoussolution. Acylation, although extremely rapid in nonpolar solvents, depends on the relative rates of reaction of the two tautomers rather than on their relative proportions. The size of the acyl group, and the steric requirements of the catalyst, are shown to be the critical factors in determining