The crystal structure of p-terphenyl has been refined using two sets of X-ray data and two projections of neutron data. The lengths of the carbon--carbon bonds, which have been determined to an accuracy of + 0-005/~, differ from those in benzene in a manner which is difficult to explain simply in terms of bond environment. The molecule departs slightly from planarity, apparently as a result of packing forces. The neutron analysis has shown that there are in-plane steric deformations of the ortho hydrogen atoms which are nevertheless 0.3/~ closer together than the sum of their van der Waals radii. The central ring oscillates more strongly about the long axis of the molecule than do the two end rings. The thermal librations are generally consistent with potential energy functions calculated for rotation about the symmetry axes of the molecule. The final X-ray difference synthesis shows features resulting from the aspherical distribution of the bonding electrons, but there is evidence of strong interaction between the bonding electron distribution and the X-ray thermal parameters. Because of this effect the difference synthesis cannot be interpreted solely in terms of the electron redistribution due to bonding, and the thermal parameters may be considerably in error.

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