a b s t r a c tThe molecular structure and conformational composition of methyl chloroacetate, H 2 ClCAC(@O) AOACH 3 , have been determined by gas-phase electron-diffraction (GED), using results from ab initio molecular orbital calculations (HF,p)) to obtain constraints on some of the structural parameters. The molecules exist in the gas-phase at 25°C as a mixture of two stable conformers: syn with CACl eclipsing C@O and gauche with CAH approximately eclipsing C@O. In both of these conformers OACH 3 is also eclipsing C@O. The experimentally observed conformational composition at 25°C was 36(8)% syn and 64(8)% gauche (parenthesised values are 2r), corresponding to a free energy difference between conformers of DG exp = 1.4(9) kJ/mol. The corresponding theoretical values obtained for DG°are 1.1 kJ/mol (HF), 2.3 kJ/mol (MP2), and 2.4 kJ/mol (MP3). The results for the principal distances (r h1 ) and angles (\ h1 ) for the major gauche conformer obtained from the combined GED/ab initio study (2r uncertainties) are r(COACCl) = 1.502(9) Å, r(CAH) = 1.084 (6) Å (average value), r(CACl) = 1.782(4) Å, r(C@O) = 1.213(4) Å, r(COAO) = 1.346(4) Å, r(CH 3 AO) = 1.468(10) Å, \CACACl = 110.0(6)°, \CAC@O = 124.7(6)°, \CACAO = 108.3(10)°, \CAOAC = 115.9(8)°, /(ClACAC@O) = 111(2)°, /(CAOAC@O) = 3(3)°.