The Z and E conformations of acetylketene (3) and its 2-methyl- and 2-chloro derivatives (6 and 8, respectively) have been obtained using ab initio molecular orbital theory at the RHF/6-31G* level, while formylketene (12) was studied at the MP2/6-31G* level. Comparison of the calculated IR spectra (RHF/6-31G*) with the experimental spectra aids in the assignment of observed absorptions, provides an estimate of the Z/E ratio, and offers an explanation of trends in the IR spectra. The relative energies were calculated at the MP2 through MP4(SDQ)/6-3lG* levels, all of which predict that the Z conformations of 3 and 8 are more stable, while the E conformation of 6 is favored, in quantitative agreement with experiment. The underlying preference for the Z conformation is due to an electrostatic attraction between the carbonyl oxygen and the central carbon of the ketene. However, steric and entropic effects combine to make E the preferred conformation of 6.