Thin films of hard, diamond-like, carbon have been deposited on glass, silicon and sapphire substrates using three different ion beam deposition techniques: (a) ion beam sputter-deposition from a graphite target, (b) sputter-deposition with simultaneous bombardment of the growing film using a second ion beam, (c) primary ion deposition using a beam extracted from an Ar-hydrocarbon plasma. Structural, mechanical optical and electrical properties were measured. It was found that the hydrogen content controlled the internal (compressive) stress and that relatively stress-free films contained ,∼ 30 atomic % H (average). The H content could be correlated with the H/C arrival ratio from the gas phase. Optical absorption and luminescence measurements gave a band gap of '∼, 1 ev with band-tailing. Resistivity measurements and their temperature dependence fitted variable range hopping models. Hardnesses were ≿ 7 Mohs. The properties of the films were consistent with a mixture of sp3 and sp2 bonds in an amorphous, hydrogenated structure.