The microstructure of hydrogenated amorphous silicon‐carbon alloys has been analyzed by small‐angle x‐ray scattering, infrared absorption, and density measurements. Decreasing density with C incorporation is due to microvoids about 0.6 nm in average radius, which are either approximately spherical in shape or randomly oriented nonspheres. The microvoid number density increases from about 5×1019/cm3fora‐Si:H to about 4×1020/cm3fora‐Si0.7C0.3:H. The CH3species probably causes the enhanced microvoid formation in these alloys. A large fraction of the microvoid surfaces is not hydrogenated.