The surface reactions during plasma enhanced chemical vapor deposition of hydrocarbon films (C : H films) from a methane electron cyclotron resonance (ECR) plasma were investigated by means of real time in situ ellipsometry. The surface reactions with atomic hydrogen leads a transformation of sp2 hybridised carbon groups into sp3 hybridised carbon groups at the growing film surface and consequently to an increase of the hydrogen content of the material. This addition of hydrogen is balanced by the release of hydrogen due to the displacement of bonded hydrogen atoms due to impinging C + and H + ions. The hydrogen release was quantified by the computer code TRIM.SP. The ion bombardment during plasma-enhanced chemical vapor deposition of hydrocarbon films leads to a disturbed surface layer, with a thickness comparable to the range of the impinging ions. This surface layer becomes apparent in the in situ ellipsometric results by optical constants, which are higher for the film surface compared to the bulk material.