B–C–N–H thin films were deposited by plasma chemical‐vapor deposition with an external capacitively coupled reactor at an rf frequency of 13.56 MHz. The films were formed from a gas mixture of B2H6(5.25 vol % in N2), methane, and argon or nitrogen as carrier gases. The deposition was carried out at room temperature and without heating the substrate. The films were transparent in the range of 10 000–2000‐A˚ wavelengths. The Knoop microhardnesses were 1825–3324 kg/mm2and the refractive index was 1.3–1.6. An extensive discussion of the effect of the deposition conditions on both microhardness and film composition is given. In addition, the interrelation between the microhardness and composition is illustrated.