Sec-butylbenzene has been investigated using resonant two-photon ionization (R2PI) and UV-UV hole-burning spectroscopy aided by ab initio calculations. All three conformers predicted from theory are observed in the spectrum, and are assigned by rotational band contour analysis. The most strongly populated conformer (G1) has a gauche arrangement of the side chain dihedral angle tau(2)(C1C alpha C beta C gamma). The populations of the anti (A) and the remaining gauche conformer (G2) are about 7% and 2%, respectively. The alpha methyl group is found to significantly affect the conformational preferences in sec-butylbenzene (sec-BB), compared to n-propylbenzene in which the anti conformer is favored. (c) 2008 Elsevier B.V. All rights reserved.