Surface processes occurring at the solid-liquid interface of Ru/TiO2 catalysts during aqueous phase reforming of ethanol at reaction conditions (200 degrees C and 20 bar) have been examined using attenuated total reflection infrared spectroscopy (ATR-IR). Depending on the loading and particle size of Ru significant differences in the population dynamics of adsorbed species were observed. On the catalyst with high loading (5 wt%) and large particle size (similar to ca. 4 nm) formation of intermediate acetylide was detected, which is attributed to enhanced C-O bond cleavage promoting methanation and thus leading to lower efficiency of H-2 generation. In contrast, on the catalyst with low loading (0.5 wt%) and smaller particle size (similar to ca. 1 nm) no acetylide was detectable, indicating that C-O bond cleavage was suppressed.