Oxidative carbomethoxylation of cyclopentene (1)under carbon monoxide and air by the use of a catalyticamount of Pd(OAc)2 and molybdovanadophosphate (NPMoV)led to dimethyl cis-1,2-cyclopentanedicarboxylate (2) anddimethyl cis-1,3-cyclopentanedicarboxylate (3) in good yields.Total yields of 2 and 3 were found to be improved by addinga small amount of NH4Cl. Several alkenes were similarlyconverted into the corresponding dimethyl dicarboxylates.The role of Cl- in the present catalytic system is suggested.