Formation of a weak aromatic C-(HO)-O-... hydrogen bond has been discerned both experimentally and computationally in the 1,2,4,5-tetrafluorobenzene (TFB)-water system. The intermolecular structure of the isolated TFB-water cluster in a supersonic jet was characterized using fluorescence-detected infrared spectroscopy. The formation of a weak hydrogen bond in the cluster was directly evidenced by a low-frequency shift and intensity enhancement of the hydrogen-bonded aromatic C-H stretch in the TFB moiety. This is the first direct observation of an aromatic C-(HO)-O-... hydrogen bond in isolated gas phase clusters. (C) 2004 Elsevier B.V. All rights reserved.