Six 4,5-di-X-9,l0-dihydrophenanthrenes (X = H, F, OCH3, Cl, CH3, CF3) were synthesized and their inversion rates were investigated by n.m.r., spectroscopy. The barriers to inversion at a standard temperature (∆G‡;340) ranged from 120 kJ mol-l and showed good correlation with the van der Waals radii of the interacting substituents at C4 and C5 and with the previously proposed 'apparent overlap' parameter.