Experiments were performed to diffuse zinc into GaAs through anodic oxide layers of varying thickness and density. Using electrochemical profiling to determine both the electrically active zinc concentration and the diffusion depth with high resolution, the following results were found. The depth of the junction varies inversely with the thickness and the density of the oxide. However, the surface concentration appears to be independent of oxide thickness or density, attaining a value identical to that found for diffusion into a bare GaAs sample. These results demonstrate that the most significant impact of the oxide is to delay the introduction of the zinc into the GaAs lattice. In short, the anodic oxide cannot be used as either a mask or as a zinc concentration attenuator.