The structures of four N,O-diacylhydroxylamines (RCOHNOCOR′, R, R′ = Me, Ph) were determined in the solid state by X-ray diffraction and studied by NMR and IR spectroscopies in solution. The interpretation of the results was supported by ab-initio calculations of various tautomers and conformers, rotational barriers and chemical shifts. The results indicate the absence of OH tautomers (R–C(OH)N–O–C(O)–R′, N-acyloxyimidic acid); the NH tautomers (R–C(O)–NH–O–C(O)–R′, O-acylhydroxamic acid) are present in DMSO solutions as equilibrium mixtures of a few conformers, their exchange being the likely source of ¹⁵N and ¹³C NMR line broadening.