The structure and conformational properties of oxalyl chloride, which experiences internal rotation about the C-C bond, have been reinvestigated by electron diffraction from the gas at 0, 80, and 190 degrees C and by extensive ab initio calculations. Complete structure optimizations at a very high level(MP2/TZ2P, 166 basis functions) revealed, in addition to the anti form at angle ClCCCl = 180 degrees, a second stable form (gauche) with angle ClCCCl = 89.8 degrees characterized by a very shallow minimum in the energy; earlier theoretical results for oxalyl chloride had been inconsistent with the existence of a second form known from experiment to be present. The electron diffraction analysis was based on dynamic models that comprised a set of pseudoconformers spaced at regular intervals around the torsional coordinate phi = angle ClCCCl and Boltzmann weighted according to a three-term torsional potential V(phi) = 1/2 Sigma(i)V(i)[1 - cos i(180 - phi)]. For the more elaborate model results from the ab initio calculations were incorporated in the form of distance and angle differences among the pseudoconformers; in a second, simpler model these differences were omitted so that the structures of the pseudoconformers differed only in their torsion angles. A theoretical force field for the anti form was also evaluated ab initio, scaled to fit the observed wavenumbers, and used in each model to calculate the usual corrections for vibrational averaging. The results for the analysis of the 0 degrees C data for the more elaborate (preferred) model are as follows (r(g)/Angstrom; angle(alpha)/deg with 2 sigma uncertainty estimates) : r(C=O) = 1.184(2), r(C-C) = 1.548(8), r(C-C> = 1.749(3), angle CCO = 123.8(4), angle CCCl = 111.8(3), and angle ClCCClgauche, = 76(18) where 0 degrees corresponds to cis; results at the other temperatures are similar. Values of the potential constants, which should be temperature independent, are found in the ranges (kcal/mol) 1.45 less than or equal to V-1 1.99, -0.40 less than or equal to V-2 less than or equal to 0.03, and 0.43 less than or equal to V-3 less than or equal to 1.05; the average values are V-1 = 1.59(83), V-2 = -0.11(38), and V-3 = 0.74(39). The estimated mole fractions of the anti form at 0, 80, and 190 degrees C are 0.67, 0.62, and 0.43, from which the internal energy difference Delta U-o = U-g(o) - U-a(o) is calculated to be 0.75(50) kcal mol(-1) and the entropy difference Delta S-o = S-go + R 1n 2 - S-a(o) to be 1.31(148) cal mol(-1) K-1. The simpler model gives similar results.