Crystals of tri-p-tolylarsine are rhombohedral, with cell dimensions a = 9.84 Å, α = 80° 2′, and space group. There are two molecules in the unit cell, and hence the molecule has symmetry C3. The structure has been determined from a projection along the rhombohedral cell axis, and the bond lengths and valency angles are given. In comparison with an ideal model having maximum interaction between the arsenic lone pair and the aromatic π-electrons, each ring is rotated about its As—C bond by 36°, the three rotations being in the same sense. These displacements increase overcrowded distances in the ideal model to about the normal van der Waals separations, the closest intramolecular contacts between p-tolyl groups beingand. All the intermolecular contacts correspond to van der Waals interactions.