The growth of oxides on , with the required properties for semiconductor applications, has obvious uses in passivation, masking, as a dielectric, etc. One promising method is anodization, and in order to assist in refining this technique, the chemistry and structure of 2500Å anodic oxides, before and after 250° and 650°C treatments in 1 atm nitrogen, have been investigated using neutron activation analysis and Auger spectroscopy combined with ion milling. The Auger data were normalized and corrected for Auger sensitivity and differential sputtering to provide profiles estimated to be quantitative within ±10%. All the oxide films investigated had compositions close to except at the interface. Hydrogen was not detectable and was ignored. The as‐anodized oxide displayed four distinct layers; these are in order, starting from the oxide surface: I , ∼400Å, highly arsenic‐deficient layer; II , ∼1700Å, moderately arsenic‐deficient layer; III , ∼300Å, arsenic‐rich transition layer; and IV , ∼400Å, arsenic‐rich interface to. The over‐all As/Ga ratio in the oxide was 0.68. After 250°C anneal, only small changes in the depth profiles were observed. After 650°C anneal, most of the arsenic and some gallium were lost, and the arsenic excess near the interface was greatly reduced.