A series of highly congested polyphenylbiphenyls and polyarylfluorenes has been prepared and their X-ray structures determined, Decaphenylbiphenyl adopts a very unusual C-1-symmetric geometry (rather than the more intuitive D-2 geometry) in which one of the central benzene rings is distorted into a boat conformation. AM1 calculations confirm that the C-1 geometry is the ground state but indicate that less highly substituted biphenyls should adopt D-2 geometries, The structure of 2,2',4,4',6,6'-hexaphenylbiphenyl supports the latter prediction; this material has crystallographic C-2 symmetry and (except for the orientation of the para phenyl groups) approximate D-2 symmetry in the solid state. Octaphenylfluorenone has been prepared in four steps from tetraphenylcyclopentadienone, Its X-ray structure shows the fluorenone core to be twisted and sterically shielded by the eight peripheral phenyl groups; nevertheless, phenylmagnesium bromide adds easily to the carbonyl group of its equally hindered dimethyl derivative, 2,3,5,6,7,8-hexaphenyl-1,4-di(p-tolyl)fluorenone, Reduction of the resulting fluorenol with TiCl3 gives a nonaarylfluorene, 2,3,5,6,7,8,9-heptaphenyl-1,4-di(p-tolyl)fluorene, and its X-ray structure shows distortions similar to those of octaphenylfluorenone.