Two conformational isomers of allylbenzene are identified in a supersonic free jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. With the aid of the predictions of ab initio quantum chemistry calculations at the MP2 level for a series of extended basis sets [6-311+G(d,p), 6-311++G(d,p), and cc-pVTZ], the major species of the electronic spectrum is shown to be an eclipsed conformer in which the allyl group is oriented perpendicular to the plane of the benzene ring and a terminal hydrogen atom of the ethylene moiety is poised nearly above the aromatic pi electrons. The minor species is identified as an internal rotational isomer that is obtained by rotating the ethylene group about the C-alpha-C-beta bond by 120degrees from the eclipsed configuration. This predicted order of conformational preference is reversed for calculations at relatively low levels of theory: MP2/6-31G(d,p), HF/6-311++G(d,p), HF/6-31G(d,p), and B3LYP/6-31G(d,p). The relative intensities of the vibronically induced nontotally symmetric and totally symmetric transitions are significantly different in the electronic spectra of the two conformers. (C) 2003 American Institute of Physics.