The rutile TiO2(110) surface is the most studied surface of titania and considered as a prototype of transition metal oxide surfaces. Reactions on flat TiO2(110)-(lxl) surfaces are well studied, but the processes occurring on the step edges have barely been considered. Based on scanning tunneling microscopy studies, we here present experimental evidence for the existence of O vacancies along the (111)R step edges (Os vac.'s) on rutile TiO2(110). Both the distribution of bridging O vacancies on the terraces and temperature-programed reaction experiments of ethanol-covered TiO2(110) point to the existence of the Os vac.'s. Based on experiments and density functional theory calculations, we show that Os vac.'s are reactive sites for ethanol dissociation via O-H bond scission. Implications of these findings are discussed. [ABSTRACT FROM AUTHOR]