The S-1((1)pi pi*) state of the (dominant) syn-conformer of 2-chlorophenol (2-ClPhOH) in the gas phase has a subpicosecond lifetime, whereas the corresponding S-1 states of 3- and 4-ClPhOH have lifetimes that are, respectively, similar to 2 and similar to 3-orders of magnitude longer. A range of experimental techniqueselectronic spectroscopy, ultrafast time-resolved photoion and photoelectron spectroscopies, H Rydberg atom photofragment translational spectroscopy, velocity map imaging, and time-resolved Fourier transform infrared emission spectroscopy-as well as electronic structure calculations (of key regions of the multidimensional ground (S-0) state potential energy surface (PES) and selected cuts through the first few excited singlet PESs) have been used in the quest to explain these striking differences in excited state lifetime. The intramolecular O-H center dot center dot center dot Cl hydrogen bond specific to syn-2-ClPhOH is key. It encourages partial charge transfer and preferential stabilization of the diabatic (1)pi sigma* potential (relative to that of the (1)pi pi* state) upon stretching the C-Cl bond, with the result that initial C-Cl bond extension on the adiabatic S-1 PES offers an essentially barrierless internal conversion pathway via regions of conical intersection with the S-0 PES. Intramolecular hydrogen bonding is thus seen to facilitate the type of heterolytic dissociation more typically encountered in solution studies.