The structure and conformation of tert-butylbenzene have been investigated by gas-phase electron diffraction, molecular mechanics (MM2 and MM3 force fields), and ab initio MO calculations at the HF/6-31G and 6-31G* levels. The theoretical calculations indicate that the coplanar conformation of the molecule, with a C-Me bond in the ring plane (0 degrees twist), corresponds to a potential energy minimum. The perpendicular conformation, with a C-Me bond in a plane orthogonal to the ring, is 2-3 kJ mol(-1) higher in energy and corresponds to a rotational transition state. The experimental study supports these findings, since the coplanar model fits the electron diffraction data better than the perpendicular one. The effective twist angle of the substituent is 8.3 +/- 2.4 degrees, corresponding to a torsional barrier V-6 = 4.7 +/- 2.7 kJ mol-l. A model with a benzene ring of C-2v symmetry and a tert-butyl group of C-s symmetry was used in the analysis of the electron diffraction data. Differences between similar bond lengths were constrained from the MM2 calculations. The following C-C bond distances were determined : ’r(g)(C-C)(ring)’ = 1.398 +/- 0.003 Angstrom, r(g)(C-inso-CMe(3)) = 1.525 +/- 0.003 Angstrom, ’r(g)(C-Me)’ = 1.544 +/- 0.003 Angstrom. The deformation of the benzene ring consists primarily of a decrease of the ipso angle to 117.1 +/- 0.3 degrees (from electron diffraction) and a lengthening of the C-ipso-C-ortho bonds with respect to C-ortho-C-meta bonds (by 0.006-0.009 Angstrom, from calculations). With regard to the effect of the benzene ring on the tert-butyl group, the C-ipso-C-Me angle lying in the ring 4plane is about 4 degrees larger than the others, and the C-ipso-CMe(3) bond is tilted from the ring axis by about 1.5 degrees.