Selective reductive coupling of benzaldehyde to stilbene is driven by subsurface Ti interstitials; on vacuum-reduced TiO2(110). A combination of temperature-programmed reaction spectroscopy and scanning tunneling microscopy (STM) provides chemical and structural information which together reveal the dependence of this surface reaction on bulk titanium interstitials. Benzaldehyde reductively couples to stilbene with 100% selectivity and conversions of up to 28% of the adsorbed monolayer in temperature programmed reaction experiments The activity for coupling was sustained for at least 20 reaction cycles, which indicates that there is a reservoir of Ti interstitials available for reaction and that surface 0 vacancies alone do not account for the coupling. Reactivity was unchanged after predosing with water so as to fill surface oxygen vacancies, which are not solely responsible for the coupling reaction. The reaction is nearly quenched if O-2 is adsorbed first-a procedure that both fills defects and reacts with Ti interstitials as they migrate to the surface. New titania islands form after reductive coupling of benzaldehyde, based on scanning tunneling microscope images obtained after exposure of TiO2(110) to benzaldehyde followed by annealing, providing direct evidence for migration of subsurface Ti interstitials to create reactive sites The reliance of the benzaldehyde Coupling on subsurface defects, and not surface vacancies, over reduced TiO2(100), may be general for other reductive processes induced by reducible oxides. The possible role of subsurface, reduced Ti interstitials; has broad significance in modeling oxide-based catalysis with reduced crystals.