Nuclear magnetic resonance spectra of solutions of acetylacetaldehyde and malondialdehyde are reported. The values of spin–spin coupling constants indicate that the enol forms exist in the s-cis-conformation in carbon tetrachloride, chloroform, and methylene dichloride, and in the s-trans-conformation in water. Evidence is provided for keto- and hydrated keto-forms of acetylacetaldehyde in solution in chloroform and water respectively.