Gas evolution experiments have been performed on hydrogenated amorphous carbon (a‐C:H) films prepared by plasma deposition. Two series of films are studied: in series I, C2H2is used as a process gas at a fixed bias voltage while the gas pressure is varied, and in series II, CH4is employed at a fixed gas pressure, and the bias voltage is changed. The results are compared to infrared absorption data and density measurements and support the presence of a void network in the amorphous material, the extent of the void network depending on the deposition conditions.