AbstractThe crystal structure of 4‐phenyl‐3‐hydroxyisoxazol‐5‐one semihydrate exhibits an exceptional hydrogen bonded polymeric structure with a unit cell of 8 molecules. The hydrogen bonds stretch out, using the oxohydroxy groups in positions 3 and 5 in the direction of one axis and along a perpendicular direction the layers are stitched together by water molecules. The layers are stitched by using four hydrogen bridges of water molecules, the heterocyclic ring nitrogen as well as the oxygen at position 5. Tautomerism of this moiety in solution is discussed, in light of some new dialkylation products. The state in which these products exist in solution depends on the solvent. A zwitterionic tautomer is present in ether. In some alkylation conditions, the predominant dialkylation product is the N,N‐disubstituted betaine (Anhydro‐2,2‐dialkyl‐3(5)‐oxo‐5(3)hydroxy‐4‐phenylisoxazolonium hydroxide). Study of tautomerism in polar and protic solvent is unreliable owing to associations a