Dynamical effects attributable to the geometric phase have been observed in UV photo-induced O-H bond fission in jet cooled phenol. Phenoxyl radicals are formed with an odd quanta progression in a '' mode nu(16a). This mode specific product formation is reproduced by 2D wavepacket simulations and discussed in terms of quantum interference resulting from the geometric phase effect at the conical intersection between the ground and first excited state potential energy surfaces at extended O-H bond lengths. (C) 2008 Elsevier B.V. All rights reserved.