We report a direct measurement of the orientation of the two CH3 groups of acetone molecule at the vapor/acetone interface. The interfacial acetone molecule is found well-ordered, with one methyl group points away around 14.4 degrees +/- 1.9 degrees and another into the bulk liquid around 102.8 degrees +/- 1.9 degrees from the interface normal, and thus the C = O group points into the bulk around 135.8 degrees +/-1.9 degrees. These results directly confirmed the highly ordered and even crystal like interfacial structure of the vapor/acetone interface from previous MD simulation. The general formulation and accurate determination of the orientational parameter D can be used to treat interfaces with complex molecular orientations. (c) 2005 Elsevier B.V. All rights reserved.