The authors report tribochemical changes due to sliding of a silicon nitride (Si3N4) ball against an ultrananocrystalline diamond (UNCD) thin film. Unidirectional sliding wear measurements were conducted for 2000 cycles using a ball-on-disk apparatus with a 3/16 in. diameter Si3N4 ball at a sliding speed of 3.3 mm/s and a normal load of 98.0 mN (nominal Hertzian stress of 0.6 GPa) in a nitrogen environment at 50% relative humidity at room temperature. The wear track produced on the UNCD film was analyzed by X-ray photoelectron emission spectromicroscopy (X-PEEM) combined with X-ray absorption near-edge structure (XANES) spectroscopy to identify and spatially resolve chemical changes inside the wear track, particularly rehybridization of carbon. XANES spectra show that SiOx complexes are deposited within the wear track. Very little rehybridization of the UNCD from its primarily sp(3) bonding configuration to sp(2) bonding is observed, and there is no observable oxidation of the UNCD, pointing to the impressive stability of the film under significant tribological loading conditions. Raman spectroscopy of the worn portion of the Si3N4 ball shows that disordered carbon is found on the worn surface. The authors attribute the formation of SiOx complexes within the wear track to wear of the ball and subsequent deposition into the track. The authors attribute the disordered carbon on the Si3N4 ball to removal of sp(2)-bonded carbon from the UNCD surface and subsequent accumulation onto the ball's surface. This work demonstrates the potential of X-PEEM for the study of tribochemical modifications of carbon-based thin film surfaces. (C) 2007 American Vacuum Society.