Alumina-silica composite films with various Al/Si molar ratios were deposited an Kapton substrate by sol-gel method and their atomic oxygen (AO) erosion resistance was tested in a ground-based AO simulator. The surface morphologies and structures of the composite films were analyzed by scanning electronic microscopy, atomic force microscopy, Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicate that with increasing the Si content, the film surfaces tend to become smooth from a particle-like morphology of the AlOOH film. The composite films are a diphase structure composed of crystal AlOOH and amorphous SiO2, where Al-O-Si bonds were detected between the two phases. The AO exposure results suggest that the composite films with Al/Si = 1/1 and 1/2 have the best AO erosion resistance. The erosion yield of the coated Kapton decreases to 2.4 x 10(-26) cm(3)broken vertical bar atom(-1), two orders of magnitude less than the value of 3.0 x 10(-24) cm(3)broken vertical bar atom(-1) of pristine Kapton. Unlike the silica film, no crack was observed in the composite films during AO exposure. However, the micro-pores in the composite films with lower Si content become the path of AO eroding Kapton substrate. As the Si content increases, the composite films provide excellent AO protection for Kapton substrate. This is due to the good toughness and densification of the composite films. (C) 2008 Elsevier B.V. All rights reserved.