A computer‐controlled rotating polarizer ellipsometer, operating in the infrared spectral region between 3.00 and 3.75 &mgr;m, has been developed forinsitucharacterization of amorphous hydrocarbon (a‐C:H) thin films, deposited from methane in a rf plasma‐enhanced chemical vapor deposition reactor. Spectroscopic IR ellipsometry permits insight into the chemical bonding structure ofa‐C:H coatings by the nondestructive detection of infrared stimulated C:H stretch vibrations. It is shown that thesp2CHx/sp3CHxratio, the content of bonded hydrogen, the infrared linewidth, and the real refractive index of the films depend on the negative self‐bias voltage, which is formed at the samples during the deposition process. A transition froma‐C:H films with polymerlike properties to harda‐C:H films was attained at a self‐bias voltage of approximately −75 V.