Both N-alkyl and O-alkyl derivatives of 2,1-benzisothiazolin-3-one (1) have been prepared by unambiguous syntheses. Comparison of ultraviolet spectra indicates that (1) exists in the keto form rather than as its tautomer 2,1-benzisothiazol-3-ol (2). Alkylation of the anion of (1) yields N-alkylated products.