The OH rotational barrier height in phenol and para-halogenophenols systematically decreases in the series Br, Cl and F, while the changes in the CO bond length and CO bond strength do not follow this order. On the basis of the NBO analysis it is suggested that lowering of the barrier height in p-halogenophenols is caused by the decrease of electron density in the sigma antibonding orbital, sigma*(CO), and a weak occupancy of this orbital is due to the electron delocalization from the sigma(CC) ring orbitals. This effect can be either reinforced or reduced, depending on the extent of conjugation between the p(z)-type lone electron pair on the oxygen atom and the pi*(CC) acceptor orbitals in the aromatic ring. (C) 2004 Elsevier B.V. All rights reserved.