Catalytic pathways of guaiacol and hydrogen (H-2) reactions on dispersed Ru clusters in the aqueous medium and the associated kinetic requirements for C-OCH3 bond cleavage and H-addition steps are established based on kinetic and isotopic investigations. Time-dependent kinetic measurements in a gradientless semi-batch reactor reveal that guaiacol reacts with H-2 via two independent routes of C-OCH3 bond cleavage and H-addition; the former leads to phenol, cyclohexanone, cyclohexanol, and cyclohexane and latter to 2-methoxy-cyclohexanol. During catalysis, an adsorbed guaiacol undergoes a single, quasi-equilibrated H-adatom (H*) addition on its aromatic ring, forming a partially hydrogenated intermediate, before the sequential kinetically relevant C-OCH3 cleavage or H* addition steps on Ru cluster surfaces nearly saturated with deprotonated guaiacol. Increasing the H* coverage promotes the overall turnovers but decreases the selectivity towards the C-OCH3 bond cleavage, because H* addition event not only activates guaiacol but also promotes H-addition without breaking its C-O bond. (C) 2018 Elsevier Inc. All rights reserved.