The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol−1. The calculated lengths of the $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ and $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Ph}}$$ bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ or $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Ph}}$$ bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are $$\langle r_g {\text{(C}}{\kern 1pt} - {\kern 1pt} {\text{C)}}\rangle {\text{ }} = {\text{ 1}}{\text{.407 }} \pm {\text{ 0}}{\text{.003 }}{\AA}{\text{, }}\langle r_g ({\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{C}})\rangle = {\text{ 1}}{\text{.881 }} \pm {\text{ 0}}{\text{.004 }}{\AA}{\text{, }}\langle r_g ({\text{C}}{\kern 1pt} - {\kern 1pt} {\text{H}})\rangle = 1.108{\text{ }} \pm {\text{ 0}}{\text{.003 }}{\AA}{\text{ }}$$ , and $$\angle {\text{C}}_{{\text{ortho}}} {\kern 1pt} - {\kern 1pt} {\text{C}}_{{\text{ipso}}} {\kern 1pt} - {\kern 1pt} {\text{C}}_{{\text{ortho}}} = 117.2{\text{ }} \pm {\text{ 0}}{\text{.3}}^\circ $$ . While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°].