Hydrogenated amorphous carbon (a-C:H) thin films were deposited onto various substrates at 200 °C by DC saddle-field glow-discharge dissociation of CH4. In situ plasma probe mass and energy spectroscopy was employed to systematically study the effects of the discharge parameters on the formation of reactive precursors. The ion current at the substrate holder consists mainly of CH3
+ for pressures < 75 mTorr; higher pressures and lower discharge currents encourage the formation of C2Hn
+ and C3Hn
+ radicals in the discharge. The growth rate was largely independent of the total pressure, increasing approximately linearly with the discharge current. Under 476 nm photo-excitation, the a-C:H films exhibited room-temperature photoluminescence in the visible, near 1.9 eV, with an intensity that was strongly dependant on the discharge parameters.