Quenched and tempered AISI 5115 steel was plasma nitrided and nitrocarburized at 550°C for 5 hours in atmospheres of 80%N2 balanced with various amounts of H2 and CO2 gases. The amount of CO2 in the gas mixture varied from zero to 10%. Optical microscopy, XRD and microhardness testing were carried out to characterize surface layers of the treated specimens. The results showed that increasing the fraction of CO2 in the gas mixture up to 7% increases the thickness of compound layer and enhances the formation of γ' phase in the layer. With regard to diffusion zone, microhardness testing did not show any change in the hardness profiles of the diffusion layers by addition of CO2 to the treating atmosphere.