The present investigation focuses on the tribological transformation occurring on a Ti alloy (Ti17) under fretting in air. Several fretting wear tests were performed on a large scale punch on plane configuration (two types of planes tested: bare Ti6242 and CuNiIn-coated Ti6242) at several temperatures from ambient up to 450 °C. In all the cases, two zones were identified on the scars: a lateral oxidized rim and a highly deformed region at centre. Metallurgical observations revealed similarities with Tribologically Transformed Structure (TTS), previously observed on various Ti alloys. In the framework of this article, careful analyses were conducted (EDX, DRX, XPS, HRTEM and EFTEM) in order to identify the nature and chemistry of this transformed layer. Results demonstrated the formation of a new phase, nanocrystalline, identified as TiOxNy. The high content of nitrogen found in the TTS indicated its ability to penetrate inside the contact and react with titanium. At 50 μm under the surface, a FIB preparation enabled the observation in TEM of N-rich lamellae (TiOxNy) in the Ti (α) matrix. Two models were suggested to explain this tribochemical reaction under fretting.