The tautomeric properties of methyl acetoacetate, CH3OC(O)-CH2-C(O)CH3, have been investigated in the Gas phase by gas electron diffraction (GED), IR(matrix) spectroscopy, and quantum chemical calculations (the MP2 approximation with 6-31G** and 6-311G(2df) basis sets and the B3LYP method with 6-31G** and 6-31 ++G** basis sets). GED results in a mixture of 80(7)% enol tautomer and 20(7)% diketo form at 309(6) K. Only one enol form with the O-H bond adjacent to the methyl group, CH3OC(O)-CH=C(OH)CH3, is present. The GED analysis cannot distinguish between the three diketo conformers which are predicted by quantum chemical calculations. IR(matrix) spectra confirm the presence of a mixture of enol and keto forms with the enol tautomer strongly prevailing. Quantum chemical calculations with the MP2 approximation predict a very small contribution of the enol tautomer, in contrast to the experiments. B3LYP calculations, however, reproduce the experimental tautomeric mixture very closely.