The adsorption and dissociation of the cis and trans isomers of azobenzene on the rutile TiO2(1 1 0) surface have been studied by means of periodic DFT. Contrary to the gas phase case, the cis isomer is found to be the most stable form on the rutile surface. The presence of the surface has an important role reducing the endothermic character of the reaction thus facilitating the breaking of the azo-bridge bond. Explicitly consideration of long-range van der Waals interactions shows that this is an important component of the interaction energy, particularly for the adsorbed trans isomer case but without changing the conclusions reached from the standard DFT calculations. (C) 2010 Elsevier B.V. All rights reserved.