Thermoplastic polyimides are reinforced with short carbon fibers (30 wt.%) and slid in a line contact over a broad interval of normal loads (50-200 N) and reciprocating sliding velocities (0.3-1.2 m s(-1)). The carbon fibers reduce the wear rates, while they either increase or decrease the coefficients of friction. The sliding mechanisms, when analysed from a mechanical point of view and considering the visco-elastic deformation, do not explain the variations in friction over the entire testing range. A new approach is introduced, considering the thermochemical modifications in the polyimide matrix. The polymer surface temperature T* is related to a product of contact pressure and sliding velocity (pv). Hydrolysis of the polyimide at intermediate pv-conditions disfavours the sliding properties of the carbon fiber composite, while imidisation under higher pv-conditions stimulates the lubricating efficiency of the carbon fibers. The formation of a monomer fraction at extremely high pv decreases the coefficients of friction due to the formation of a mixed transfer film. (C) 2008 Elsevier B.V. All rights reserved.