A low-alloy steel was nitrided in the downstream zone of an electron cyclotron resonance (ECR) discharge at low pressure (0.15-0.25 Pa). Structure and properties of the nitrided surfaces were primarily controlled by the substrate temperature T-s (300-550 degreesC), the nitrogen-hydrogen-argon gas mixture composition, and the substrate bias voltage U-b (-1000 to +40 V) at a typical value of the incident microwave power P-i = 900 W, the distance of the substrate from output aperture of the ECR source was d = 250 mm, and the nitriding time t = 2 h. Optical emission spectroscopy was used to study the behavior of significant atomic and molecular species, such as N-2, N-2(+), NH, H, Fe, Ar, and Art, in front of the nitrided sample. It has been found that due to a high plasma reactivity, nitriding is effective at substrate temperatures T(s)greater than or equal to 500 degreesC, when the surface hardness is about 1200 HV0.05 and a diffusion layer thickness up to 120 mum has been achieved for t = 2 h. The presence of 10%-30% H-2 in a nitrogen-hydrogen gas mixture enhances the efficiency of nitriding in comparison with treatment in pure nitrogen under the same conditions. For T(s)greater than or equal to 500 degreesC, the process is effective even for substrates at positive potential (U-b = + 30 V) and for a lower amount of N-2 (10%-30%) in a nitrogen-hydrogen-argon gas mixture, for which no compound layer is formed on the nitrided surface. However, the negative substrate bias voltage U-b enhances considerably the efficiency of nitriding only at lower substrate temperatures (Ts approximate to 400 degreesC), when the nitriding is relatively weak.