The tautomerism of the aliphatic MeC(NH₂)NOH and the aromatic PhC(NH₂)NOH amidoximes in protic (H₂O, MeOH) and aprotic (Me₂CO, Me₂SO, and CHCl₃) solvents was studied by density functional theory calculations at the M06-2X/6-311+G(d,p) level of theory. In both types of solutions, these species exist in 3 tautomeric forms, viz, (Z)- or (E)-amidoximes, and (Z)-aminonitrone. The (Z)-form is the dominant, and energy gap between the (Z)- and (E)-tautomers of the amidoximes slightly depends on the nature of a solvent and significantly higher than that for other oxime species. For the amidoximes, the zwitterionic (Z)-aminonitrone form is stabilized by protic solvents. The oxime-nitrone energy gap is reduced by electron-donor substituents at the oxime moiety at different types of oxime species.