Plasma immersion ion implantation (PIII) of Ti and Ti6Al4V alloy in dry air plasma has been performed with 25 W negative pulses up to 1.9 X 10(18) cm(-2) doses. For comparison, prolonged (50- 100 h), high-temperature (600-650 degreesC) heat treatment of a similar Ti-alloy in air (TO treatment) was also performed. The changes in chemical composition, structure and hardness of the modified surfaces were studied by XPS, X-ray diffraction (XRD) analysis and nanoindentation measurements. According to XPS, surface oxidation and strong surface enrichment of At occurred on the Ti-alloys after both the "non-equilibrium" PIII treatment and the "equilibrium" TO treatment. After the air PIII treatment Ti and Al were present in fully oxidized (TiO2 and Al2O3) States, and neither nitrogen nor vanadium could be detected in the topmost layer. XRD showed the formation of rutile and substoichiometric TiO2-x phases on the PIII-treated Ti and TO-treated Ti-alloy, but no crystalline oxide phase was found on the PIII-treated Ti-alloy. The surface hardness and the scratch resistance of the samples increased significantly after PIII treatment. The surface hardening and the improved scratch resistance of the oxidized Ti-alloy samples can be explained mainly by the surface segregation of Al and the formation of a layer containing oxidized Ti and Al.