The use of mixed solvent of N-methylpyrrolidone (NMP) and water and of mixed base of triethylamine and sodium carbonate was effective for promoting the rate of homogeneous Heck reaction of iodobenzene and methyl acrylate with palladium acetate as catalyst in the absence of any ligands. II was observed that palladium precipitated and formed colloidal particles during the reaction and this was promoted by the presence of water. However, the reaction proceeded with palladium species dissolved in the solvent, which were more active than those in pure NMP solvent. When the same Heck reaction was conducted with an activated carbon-supported palladium catalyst, the presence of water was also effective for promoting the rate of reaction. It was observed that palladium was leached out from the support and the dissolved palladium species were also active species for this heterogeneous reaction. The presence of water seemed to suppress the leaching of palladium into the solvent and/or enhance the formation of colloidal palladium particles. The concentration of active palladium species was smaller in the mixed solvent than in pure NMP solvent. However, these were more active, resulting in the larger rate of reaction in the presence of water.