The metal ion effect on the bulk VPO was investigated using the aldol condensation of methyl acetate and formaldehyde to methyl acrylate. The VPO catalyst with 2 mol % Zr exhibited the highest methyl acrylate formation rate of 18.6 mu mol.g(-1).min(-1). A combination of X-ray diffraction, BET, TPD, XPS, and EPR, together with activity testing data, was utilized to analyze and explain the origin of the metal ion effects. The promotion of methyl acrylate production by the intercalation of Zr was associated with its interaction with the VPO phase. When Zr concentration attained 2 mol %, the Zr4+ ion was distributed in the surface adjacent layers and partially incorporated into VPO species, giving rise to a higher fraction of the beta-VOPO4 entity and a relatively higher amount of the badly distorted (VO)(2)P2O7 entity. These findings suggested that both beta-VOPO4 and distorted (VO)(2)P2O7 phases were required for the formation of methyl acrylate.