The mechanism of phosphine-catalyzed hydroalkoxylation of the methyl vinyl ketone has been investigated by the second-order Moller-Plesset perturbation theory and the conductor-like polarized continuum model. The free energy reaction profiles of the reaction in both gas phase and solution phase are explored and compared. Our results suggest that the first stage of the studied reaction is the generation of the base (the methoxide anion) with the help of trialkylphosphine, and the second stage is the hydroalkoxylation of the rnethyl vinyl ketone catalyzed by this base. In the first stage, trialkylphosphine first adds to the methyl vinyl ketone to form a phosphonium enolate intermediate and then this species deprotonates a methanol molecule to generate a methoxide anion. Both steps involve free energy barriers of about 20 kcal/mol. In the second stage, both the addition of the methoxide anion to the rnethyl vinyl ketone and the proton transfer process from methanol to the methoxyl enolate anion intermediate have activation free energies of about 16 kcal/mol. The reaction in the second stage is exothermic by 10.2 kcal/mol at room temperature. A comparison of the free energy reaction profiles in the gas phase and the solution phase demonstrated that the generation of the methoxide anion could only occur in the presence of the polar solvents. The mechanism proposed in the present work is in reasonable agreement with the known experimental facts.