Hydrogenated amorphous carbon (a‐C:H) films, deposited onto negatively biased substrates in a 13.56‐MHz hydrocarbon glow discharge system, have been investigated by mass spectroscopic thermal effusion measurements. Depending on the bias voltage UB as the most important deposition parameter, hydrocarbons and/or H2 molecules are desorbed at threshold temperatures between 300 and 600 °C. The threshold temperature increases with increasing bias voltage while the mass of the desorbed molecules decreases. a‐C:H films deposited at low bias release H2, CH4, and higher hydrocarbons whereas from hard a‐C:H films deposited at UB≳500 V, only H2 is released. Using double‐layer a‐C:H/a‐C:D films, it is shown that H2 and CH4 molecules are formed in the volume of the film followed by molecular diffusion through the a‐C:H network. For high‐bias voltages (UB≳500 V), the reduced pore size of the strongly crosslinked a‐C:H network is shown to prevent diffusion of hydrocarbon molecules, while the films are still permeable for hydrogen.