A systematic study was undertaken with samples of Ti-6Al-4V nitrided in a unusual RF plasma equipment using a N-2-H-2 gas mixture under 10 Pa. As treatment parameters, we have used: nitriding time (from 30 to 720 min) and samples temperature (from 500 to 900 degreesC). In order to evidence the plasma efficiency, thermal treatments of nitridation, assisted or not by RF plasma, were performed in the same conditions. The nitrided titanium specimen were characterized by XRD, SEM, TEM, EELS, and microhardness measurements. The plasma is shown to enhance the formation of richer compounds like delta-TiN and epsilon-Ti2N nitrides on the surface at lower temperature than that obtained with a classic thermal treatment. Moreover, it leads to a significant reduction of the residual compressive stress. At moderate temperature (700 degreesC) and for longer treatments, the plasma treatment enables the formation of a delta-TiN layer which is thought to act as a diffusion barrier for nitrogen. Then, the underneath previously formed epsilon-Ti2N grains undergo a nitrogen rearrangement with the remaining alpha-Ti grains leading to the formation of a nitrogen poor phase that was identified as the alpha-TiN0.26 phase. (C) 2003 Elsevier B.V. All rights reserved.