Dual‐mode microwave/radio frequency plasma‐enhanced chemical‐vapor deposition allows one to decouple ion bombardment effects from processes in the discharge volume. This approach has been used to deposit three types of hydrogenated amorphous films at low substrate temperature and high deposition rate (∼10–20 Å/s): SiN x , SiO2, and a‐C:H. For each of these materials, we have determined critical values of the negative bias potential, V B,C , of the average ion energy, Ē i,c , and of the ion/condensing‐atom flux ratio (φ i /φ n ) c , which characterize the transition from a porous to a densely packed microstructure. The evaluations are based on measurements of the films’ resistivity, dielectric loss tangent, microhardness, density, and stress. The Ē i,c , (φ i /φ n ) c values found are: 170 eV, 0.60 for SiN x ; 70 eV; 0.26 for SiO2; and 80 eV, 0.28 for a‐C:H. Ion bombardment at energies above Ē i,c has been found to account for a large portion of hydrogen in the films which is not chemically bonded. The results are interpreted in terms of a growth process involving surface diffusion of precursor species, and subplantation in hydrogen‐rich surfaces.