Ultrathin fluoropolymer films were deposited by a plasma-enhanced chemical vapor deposition process, from mixtures of hexafluoropropylene (C3F6) and octafluoropropane (C3F8) precursors. Different monomer feed compositions result in different rates of deposition. The C3F6 is the primary polymerizing species and C3F8 acts to reduce the deposition rate. The structure and composition of these films were characterized using Fourier transform infrared and angle-resolved x-ray photoelectron spectroscopy. The bulk F:C ratio is 1.1 for the homopolymer film (deposited from C3F6 only) and 1.4 for the copolymer film (deposited from C3F6 + C3F8 monomer feed). The copolymer film has more -CF3 groups and has a more highly branched structure. The surface F:C ratio is 1.5 for the homopolymer and 1.7 for the copolymer film. The water contact angle increased with film thickness and with addition of C3F8 to the monomer feed. A mechanism is presented to describe how the addition of C3F8 modifies the polymerization process, causes a high surface F:C ratio of the copolymer films, and leads to reduced surface energy.