The reaction paths leading from ethyl propiolate to isoxazolin-5-one (II) and isoxazolin-3-one (III) are discussed and shown to be pH-dependent. The chemical and tautomeric properties of compound (II) are consistent with the known behaviour of its derivatives. The apparent dissociation constants of (II) and of its derivatives are analysed in terms of contributions from the tautomers present in aqueous medium, as ascertained from u.v. data; substitution at position 4 appears to cause a large increase in the acidic strength of Δ2-isoxazolin-5-one tautomers but to have no marked effect upon that of Δ3-isoxazolin-5-one tautomers. The tautomeric equilibria of (II) in several solvents have been studied by u.v. and n.m.r. spectroscopy and compared with the known behaviour of derivatives of (II); consistent with the findings on acidity, substitution at position 4 is shown to increase the relative stability of Δ3-structures in comparison with the corresponding Δ2-structures.