Chemical analysis of the interface between two contacting materials, before and after a wear test, is essential for a complete ลฝ . understanding of the tribological process. X-ray photoelectron spectroscopy XPS is a powerful technique for the analysis of surfaces and for this reason, we have developed a tri-pin-on-disk apparatus to run within the controlled environment of an XPS spectrometer. The rig can run at variable speeds, with a normal force between 6 and 42 N on each 5-mm diameter pin, giving an apparent maximum pressure of 2 MPa, and, therefore, a wide range of conditions. The normal force is measured before the disk is lowered on to the pins and the friction force can be recorded throughout a wear test. At the end of a wear test, worn pins are transported to the analysis chamber of the XPS spectrometer without contamination from the atmosphere. Experiments have been carried out to investigate the extent and chemical ลฝ . ลฝ . composition of transfer films after tests with PEEK poly-ether-ether-ketone and APC2 a PEEK-bonded carbon fibre composite sliding ลฝ against 316 stainless steel. No chemical change was found for PEEK pins sliding against fine ground stainless steel disks Ra 0.3 " 0.02 w x . ลฝ y8 . mm optical . For steel pins sliding against an APC2 disc in vacuum 10 mbar , relatively thick layers of amorphous carbon were transferred to the steel after only a short period of wear. A similar experiment in air produced thinner carbon transfer films. The apparatus provides a convenient way of measuring chemical changes that may occur on surfaces undergoing wear and provides a new capability for EPSRC Users of the Scienta ESCA300 XPS at Daresbury Laboratory.