The structure and the conformational composition of chloromethyl chloroformate, ClC(=O)-O-CH2Cl, has been studied by using gas-phase electron diffraction (GED), ab initio molecular orbital calculations, and earlier published vibrational spectroscopic data. The majority of the molecules [94(6)%] have a syngauche conformation where the -CH2Cl group is syn to the carbonyl bond [torsion angle phi(O=C-1-O-2-C-3) approximate to 0 degrees] and CH2-Cl is gauche to the C-1-O bond [torsion angle phi(Cl-C-3-O-2-C-1) = 83.5(19)degrees]. The second conformer is a syn-anti form where the -CH2Cl group is also syn to C=O but where CH2-Cl is anti to C-1-O [phi(Cl-C-3-O-2-C-1) = 180 degrees]. Assuming the entropy difference between the two conformers obtained from ab initio calculations [MP2/6-31G(d)], this composition corresponds to an energy difference of Delta E degrees = 1.7(7) kcal mol(-1) The experimental bond distances (r(g)) and bond angles (angle(alpha)) of the major syn-gauche conformer, with estimated 2 sigma uncertainties (sigma includes estimates of uncertainties in voltage/camera distance and of correlation in the experimental data) are : r(C-H) = 1.097(14) Angstrom, r(C=O) = 1.193(2) Angstrom, r(C-1-O) = 1.348(3) Angstrom, r(C-3-O)= 1.416(4) Angstrom, r(C-Cl)= 1.745(2) Angstrom, r(C-3-Cl) 1.777(2) Angstrom, angle O-C=O = 126.8(3)degrees, angle O-C-1-Cl = 108.9(3)degrees, angle O-C-3-Cl= 111.4(5)degrees, angle C-O-C = 117.8(7)degrees, angle O-C-H= 108.3(17)degrees; a vibrational force field was evaluated by symmetrizing the quantum-mechanical [MP2/6-31G(d)] Cartesian force constants and scaling the results to fit the observed vibrational wavenumbers.