Car-Parrinello molecular dynamics combined with metadynamics simulations were used to study the acid-catalyzed hydrolysis of cellobiose (CB) in aqueous solution. The hydrolysis was studied in two steps. Step 1 involves the proton transfer from solvent to CB and dissociation of the glycosidic bond to beta-glucose and oxacarbenium ion species. Step 2 involves the formation of alpha-glucose from oxacarbenium and regeneration of the acid proton species. Step 1 is endothermic, while Step 2 is exothermic. The overall activation free energy of CB hydrolysis is 32.5 kcal mol(-1), and the overall reaction free energy is -5.9 kcal mol(-1), consistent with available experimental data. We observe that a stepwise mechanism generally described in the literature for Step 1 is not significantly favored relative to a concerted beta-1,4' linkage dissociation process.