The structure of the liquid/vapor interface of acetone is investigated theoretically by molecular dynamics simulation and experimentally by infrared-visible sum frequency vibrational spectroscopy. The simulation yields the molecular orientational distribution at the liquid surface. In conjunction with the ab initio calculation, the sum-frequency vibrational spectra for the interface is then computed, allowing a direct comparison to the measured spectra. As the calculation agrees well with the experiment, we can conclude that the molecules at the liquid surface are polar ordered, with one of the methyl, groups pointing away from the bulk and the molecular plane perpendicular to the surface. This orientation is similar to that of molecules in the layer planes of crystalline acetone. Together with the appreciable surface tension of acetone, it suggests that the acetone liquid surface has a more ordered structure than the bulk and is more or less crystal-like. (C) 2001 American Institute of Physics.