The epoxidation rate of alpha, beta-unsaturated acids catalyzed by Na2WO4 or Na2MoO4 in H2O has a close relationship with pH of the solution. The epoxidation rate is the highest at pH 5.0-6.0. A quantum chemical ab initio method (MP2/6-31G** level) was used to calculate the molecules and anions of some alpha, beta-unsaturated acids. The results indicate that the orbital energy of MO which bonding pi orbital of double bond in monoanion lies in is a determinant factor for the rate. If pH is low (pH < 3), alpha, beta-unsaturated acid exits mainly in a molecule state and the bonding pi orbital of double bond usually lies in HOMO. The energy of HOMO is very low, in the range of -10 to -12eV, so it is difficult to be epoxidized. If pH is in the range of 4.0-6.5, the organic acids mainly exit in monoanion state, and the orbital energy of the MO which bonding pi orbital in the monoanion lies in is in the range of -4.5 to -6.5 eV. Increase of the energy is about 5 eV that is large enough to increase reactivity of double bond to initiate the epoxidation. If pH > 7, though molecules can exist as a dianion for dicarboxylic acid or dibasic acid, the epoxidation is almost impossible. Therefore, the epoxidation of alpha, beta-unsaturated acid is to be carried out in their monoanionic states. The results support the direct oxygen transfer mechanism suggested by Sharpless. (C) 2004 Elsevier B.V. All rights reserved.