The correlation between composition, microstructure, and optical properties of a‐Si_1−xC_x:H thin films with different stoichiometries was established. The alloys were deposited by radio frequency glow discharge under ‘‘starving’’ plasma conditions from mixtures of SiH₄ and CH₄. The samples were characterized by small angle x‐ray scattering, ultraviolet‐visible and infrared spectrometry, and Auger electron spectroscopy. The results showed the presence of microvoids with sizes between ≂3 Å and ≂8 Å. The relative microvoid volume fraction displayed a maximum for x around 55 at. % and decreased for higher values of x. High carbon content alloys (x≂70 at. %) not only have a lower relative microvoid volume fraction, but show optical gaps as high as 3.7 eV, high resistivity, and very low refractive index, indicating the presence of a diamond‐like C‐C structure. These remarkable results are attributed to the deposition under ‘‘starving’’ plasma conditions.