Thin films (less than 2000 Å) of amorphous hydrogenated carbon have been characterized by neutron reflectivity measurements. Films were deposited onto silicon substrates from a methane r.f. plasma to produce both very hard and very soft materials. The critical angle and density give the film composition (CH0.55±0.05 hard film, CH0.78±0.08 soft film). Fitting the interference pattern to a model gives the film thickness and a measure of the extent of mixing at the silicon-carbon interface. A structural model for plasma-deposited carbon films is used to correlate film composition with plasma deposition conditions and optical band gap measurements.