Diamond thin films grown by the oxy-acetylene flame method have been investigated by x-ray absorption and Auger electron spectroscopies. The quality of the films, defined as the ratio of diamond to non-diamond carbon, has been determined by using both techniques, giving values as high as 95% for the sample grown with the highest C2H2/O-2 molar ratio (0.97). Values obtained by Auger electron spectroscopy are slightly lower due to the smaller probe depth of this technique as compared with x-ray absorption spectroscopy (5 Angstrom in the first case against 50 Angstrom in the latter), and to the higher non-diamond carbon content of the surface, as it is expected from the mechanisms of diamond growth by this method.