Dissociation of ascorbic acid in water has been studied by using a cluster model. It was examined by density functional theory (DFT) with the В3LYP, M06, and wB97XD functionals and a 6-311++G(d,p) basis set. The thermodynamic and kinetic characteristics of proton transfer from ascorbic acid molecule to water clusters were calculated as well as the equilibrium constants (pK a ) for the related processes. The used functionals in the DFT method together with continuum solvent models provided results close to the experimental data for the dissociation constant of ascorbic acid in aqueous solution.