Experimental and theoretical studies were performed to investigate the glycerol hydrogenolysis to 1,2-propanediol (1,2 PD) over Cu/Al₂O₃ and the alumina hydration effect on catalytic activity. The experimental results show that glycerol hydrogenolysis on Cu/Al₂O₃ is highly active and selective to 1,2 PD, and water contained in the glycerol feedstock can decrease the catalytic reactivity. The DFT calculations demonstrate that the improved catalytic activity on Cu/Al₂O₃ compared to pure copper catalyst is assisted by the acidic sites (Al sites) of the alumina surface and its partial hydration. The alumina hydroxylation significantly modifies its Al site activity. The Al site and the copper site on hydroxylated alumina show similar affinity for glycerol and acetol adsorption. The Al site shows lower barrier energy of the glycerol initial O–H bond cleavage than the Cu site. However, water contained in reactants additional to water produced during the dehydration step may initially reduce the number of active Al sites, consequently, the catalytic reactivity decreases.