Al-C-N-O composite thin films have been synthesized by radio frequency reactive diode sputtering of an aluminum target in plasmas of N-2 + O-2 + CH, gas mixtures. The chemical structure and composition of the films have been investigated by means of infrared and X-ray photoelectron spectroscopy. The results reveal the formation of C-N, Al-C, Al-N and Al-O bonds. The X-ray diffraction pattern suggests that the films are of nanometer composite material and contain predominately crystalline grains of hexagonal AlN and alpha -Al2O3. A good thermal stability of the composite has been confirmed by the annealing treatment at temperatures up to 600 degreesC.