The effect of water added to monomer being supplied in the vapor phase was investigated in photografting on sensitizer-coated polymer films at 60°C. A pertinent quantity of water accelerated sharply the photografting irrespective of the nature of monomer (acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, and acrylonitrile), film substrate (low-density and high-density polyethylenes, polypropyrene, nylon 6, and polyester), and sensitizer (benzophenone and anthraquinone). Water was found to increase the molecular weight of the grafts, which was studied on acrylonitrile-grafted nylon 6 film. It was confirmed that water exhibited an accelerating action on photoinduced solution polymerizations of acrylic acid and methyl methacrylate, resulting in an increased molecular weight of the both polymers. These results suggest that the accelerating action of water may result from the restriction of chain termination.