Reactions of bromobenzene and chlorobenzene with methyl acrylate were carried out in N-methylpyrrolidone (NMP) over a conventional Pd/C catalyst under Heck coupling conditions indicating both heterogeneous and homogeneous character. Benzene and biphenyl are produced in addition to the Heck coupling product of methyl cinnamate. It is proposed that the formation of methyl cinnamate and biphenyl proceed homogeneously with dissolved palladium species, while the formation of benzene takes place heterogeneously on the surface of supported palladium particles or free colloidal particles formed during the reaction.