The tautomeric and conformational properties of malonamic acid methyl ester, NH2C(O)-CH2-C(O)OCH3, have been investigated by means of gas-phase electron diffraction (GED) and quantum chemical calculations (HF, B3LYP, and MP2 approximations with different basis sets up to 6-311++G(3df,pd)). Both quantum chemistry and GED at 360(8) K result in the existence of a single diketo conformer in the gas phase. According to GED refinement, this conformer possesses an (ac, sc) conformation with dihedral angles C-C-C(NH2)=O of 140.3(3.0) degrees and C-C-C(OCH3)=O of 31.1(7.2) degrees. The experimental geometric parameters are reproduced very closely by MP2 and B3LYP methods with large basis sets.