Hydrogenated amorphous (a‐C:H) films were prepared by rf‐plasma deposition from benzene vapor. Complete optical absorption spectra from the UV to the IR (0.2–20.0 &mgr;m) have been measured. The optical gap depends linearly on hydrogen content andEoptcan be varied between 0.8 and 1.8 eV. For energies belowEoptthe films are almost transparent and absorption is especially low in the 2–6‐&mgr;m region (e.g., &agr;=15 cm−1at &lgr;=2.8 &mgr;m). Sharp C–H stretch absorption bands occur near 3.4 &mgr;m, giving insight into the microstructure of the films. A newly reported weak band at 3.03 &mgr;m is first evidence for C–C triple bonds (sp1hybridization) ina‐C:H, wheresp3(single) andsp2(double) C–C bonds dominate.