A model for chemical vapor homo- and copolymerization is presented in two dimensions, allowing determination of order of initiation in homopolymers and reactivity ratios in copolymers. The model assumes a linear pressure gradient along the flow direction and allows calculation of the order of initiation if thickness vs position are known, and r(1) and r(2) if the gas flow rates, final film thickness, and film composition vs distance are known. Experiments indicate the number, Lambda, of p-xylylene molecules required to initiate polymerization : For poly(p-xylylene) and poly(p-chloroxylylene), Lambda = 3; for poly(p-dichloroxylylene), Lambda = 4. Tentative values for the reactivity ratios of poly(p-chloroxylylene) with perfluorooctyl methacrylate have been determined at different temperatures. At 20 degrees C, r(1) = 13.04 and r(2) approximate to 0; r(1) increases with increasing temperature, while r(2) remains vanishingly small.