The design, criteria, construction and operating features of a gaseous plasma reactor to accomplish reactions of partially ionized gases with the surface of polymer substrates, either films or individual parts, is described. The mode of gas excitation is by capacitive electrodeless r.f. energy from conventional r.f. generator sources and impedance matching networks. In this apparatus the conditions within the plasma can be maintained over powers from minimum to 500 w and pressures from several millitorr to several torr. Values of power and pressure can be selected within this range to ensure that the bulk properties of the polymer are not compromized. Internal reflectance infra-red evidence is presented for types of surface reactions that can be produced on several polymer surfaces.