The mechanism and energetics of furan decomposition via the opening of the five-membered ring structure in dilute acid solution were investigated using density functional Car-Parrinello molecular dynamics combined with metadynamics simulations. Diffusion of an acidic proton from the aqueous medium leading to the formation of a protonated furan is found to be the rate-limiting step of the ring-opening process, with protonation at the C-alpha position being 7 kcal mol(-1) less activated than that at the C-beta, position. Protonation at the C-alpha position results in the formation of 2,5-dihydro-2-furanol or 2,3-dihydro-3-furanol via nucleophilic attack by a solvent molecule. Subsequent protonation of the furanols at the ring oxygen initiates the opening of the furanic ring, leading to the formation of 4-hydroxy-2-butenal.