Characteristics of diffusion layers on the steels after the plasma electrolytic nitricarburising (PEN/C) depend on the electrolyte composition. Features of the anode PEN/C are an oxide layer formation and the anode dissolution. The oxide layer is established to inhibit the carbon diffusion [1]; this layer thickness is determined by the rate of oxidation and dissolution which occur in parallel. This study is devoted to the influence of the electrolyte components concentrations (ammonia chloride and carbamide) and anode PEN/C conditions on the properties of the treated surface. As the saturating component an inexpensive carbamide is used this is capable of producing nitrogen and carbon. Hypothesis of the thermal decomposition of the carbamide with the isocyanic acid formation is confirmed in this study by the isocyanate ions detection in the electrolyte. Ammonia is a nitrogen source in the vapour-gas envelope and monoxide carbon is a carbon one.  Anode PEN/C of steel in the above solution provides to formation of the porous oxide layer, the external nitrocarburised layer, and internal diffusion layer. According to data of X-Ray diffraction ferrite, martensite, retained austenite, iron oxides as well as iron nitride FeN_0.05 with low nitrogen concentration were found in the surface layer of the samples as a result of anode PEN/C with following quenching. The external nitrocarburised layer contains the nitride, pearlite, and martensite with retained austenite depending on the cooling condition. According to data of EDX analysis the increased nitrogen concentration (0.5–1.6 wt. %) in the external nitrocarburised layer characterizes the presence of the disperse iron nitrides surrounded by the solid solution of carbon and nitrogen in the ferrite and pearlite grains. The diffused carbon is displaced off the surface by nitrogen; therefore the carbon concentration maximum is at some depth from sample surface. The internal layer is a solid solution of carbon and nitrogen where their concentrations are higher than the initial one. Maximal hardness observed at the depth 40–50 mm is supposed to be related to the highest total concentration of nitrogen and carbon which form their supersaturated solution in the austenite at the processing temperature and martensite phase following the cooling in electrolyte.   It is established that the concentration rise of the ammonia chloride promoted to anode dissolution leads to the decrease in the oxide layer thickness and the increase in the nitrocarburising one. Hence, the oxide layer inhibits the nitrogen diffusion in a manner similar to carbon one. On the contrary, the increase in carbamide concentration results in the growth of all layers. The results obtained show that increase in the carbamide concentration promotes to the more intensity diffusion of carbon and nitrogen in steel, and this factor exceeds the inhibiting role of the oxide layer.   Increasing in the ammonia chloride concentration leads to decreasing in surface roughness that is associated with the increasing in the current density and, respectively, the anode dissolution rate. The surface roughness rises with the processing temperature that can be explained by increase in the rate of the surface oxidation and formation of the porous oxide layer which is distributed irregularly on the sample surface. Smallest roughness R_a = 0.11 mm was obtained at 750°С and 5 min with the use of the electrolyte contained 10 wt. % carbamide and 10 wt. % ammonia chloride.