The molecular structure and conformational composition of 1,1-dichloro-2-propanone (1,1-dichloroacetone) have been investigated by gas-phase electron diffraction at 60 degreesC. Results from ab initio molecular orbital calculations were used as constraints in the analysis. The experimental data are consistent with a model where the molecules exist predominantly with the hydrogen synperiplanar to the carbonyl group. Small amounts (up to 15%) of a second anticlinal form (C-Cl eclipsing C=O) cannot be ruled out. Ab initio calculations showed that both synperiplanar and anticlinal forms are stable conformers with the former conformer lower in energy by 2.25 kcal mol(-1) (MP2/6-311+G(d)). The geometrical parameters for the synperiplanar conformer (r(g) and <(alpha) obtained from least squares analyses of the electron diffraction data, using results from the theoretical calculations to provide some constraints in the model used, are r(C-H)(av) = 1.093(14) Angstrom (average value), r(C=O) = 1.196(6) Angstrom, r(C-1-C-2) = 1.524(4) Angstrom, r(C-2-C-3) = 1.496(4) Angstrom, r(C-1-Cl)(av) = 1.775(2) Angstrom, <(CCC1)-C-3-C-2 = 115.2(5)degrees, <(CCO)-C-1-O-2 = 119.6(5)degrees, <(CCO)-C-3-O-2 = 125.2(5), <(CCClav)-C-2-Cl-1 = 110.6(5), phi((HCCO)-C-1-O-2) = 2(8)degrees, and phi((HCCO)-C-3-O-2) = 25.6degrees (ab initio value).