Reductive coupling to form stilbene is the dominant reaction of benzaldehyde on TiO2(001) single-crystal surfaces reduced by argon ion bombardment. Stilbene yields of 56% of the adsorbed benzaldehyde layer and selectivities up to 72% of the benzaldehyde reacted were obtained in temperature programmed desorption experiments under ultrahigh vacuum conditions. Reductive coupling requires a reduced surface. The stilbene yield;decreased by an order of magnitude when the TiO2(001) surface was oxidized by annealing at 850 K prior to benzaldehyde adsorption. The extent of surface reduction was determined from the populations of titanium cations in different oxidation states quantified by X-ray photoelectron spectroscopy. The activity of the surface for reductive coupling was found to track the extent of surface reduction as the latter was varied either by annealing or ion-bombardment procedures. However, in contrast to the mechanism commonly proposed for reductive coupling of aldehydes with titanium-based reagents in liquid-solid slurries, no Ti-0 was observed by XPS even on the most active surfaces. The active site required for gas-solid reductive coupling is an ensemble of Ti cations in the +1, +2, and +3 oxidation states which collectively effect this four-electron reduction. These results demonstrate novel carbon-carbon bond formation on oxide surfaces in UHV and illustrate the application of surface analytical techniques to probe the sites of organic synthesis at the fluid-solid interface.