Despite a significant body of literature on the photocatalytic decomposition of 4-chlorophenol by TiO2 at liquid/solid and gas/solid interfaces, a fundamental understanding of the interaction of 4-chlorophenol with TiO2 is lacking. We present the first study of this interaction under well-defined UHV conditions by means of NEXAFS, time-dependent XPS, and UPS. XPS data show that the molecule adsorbs with the carbon framework intact and no scission of the C-Cl bond. The NEXAFS results indicate a coverage-dependent tilted geometry for the adsorbed molecule that is attached to the surface via a phenolate link. In contrast, because of the absence of an OH functionality, 1,2,4-trichlorobenzene lies flat. The adsorption of 4-chlorophenol is accompanied by a decrease in the TiO2 work function pointing to adsorbate -> substrate charge transfer. Interestingly, the UPS data suggest that 4-chlorophenol adsorption leads to the photosensitization of TiO2, thus providing a basis for understanding recent results on the visible light photocatalytic activity of TiO2 for 4-chlorophenol decomposition. It is also found that the molecule is stable against thermal decomposition at temperatures of up to similar to 473 K, which is well above the temperature range used for photocatalytic decomposition studies.