The molecular structure of p-diethynylbenzene has been determined by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G* and MP2/6-31G*(fc) levels. The two ethynyl groups undergo large-amplitude bending motions, making the equilibrium D-2h model inadequate to describe the average structure from electron diffraction. Based on spectroscopic information on low-frequency modes, the electron diffraction data were approximated by a model consisting of a mixture of rigid conformers, differing only in the extent of the symmetric out-of-plane bending of the substituents. This gave the following geometrical parameters : angle C-ortho-C-ipso-C-ortho = 119.2 +/- 0.2 degrees, [r(g)(C-C)] = 1.402 +/- 0.003 Angstrom, r(g)(C-ring-C-sp) = 1.431 +/- 0.003 Angstrom, and r(g)(C = C) = 1.211 +/- 0.003 Angstrom. The computed r(e) values (MP2) are 119.2 degrees, 1.401 Angstrom, 1.430 Angstrom, and 1.223 Angstrom, respectively, with the C-ipso-C-ortho bond 0.016 Angstrom longer than the central C-C bond. The HF/6-31G* geometries of ethynylbenzene and p-diethynylbenzene indicate that the interaction of the ethynyl group with the ring is not affected by the presence of another ethynyl group in the para position. Comparison with solid-state results shows no appreciable effect of crystal environment on the ring deformation in these molecules.