Zn‐doped GaAs diffused with Sn has been examined in cross section by transmission electron microscopy (TEM) and high‐resolution cathodoluminescence (CL) in an attempt to account for the penetration of Sn into this material but the apparent lack of any electricity activity. TEM shows that small Sn‐rich precipitates are present in the GaAs near the surface. CL indicates that some of the Sn dopes of GaAsn‐type, but additionally that new emissions in the spectra at ∼1.44 and ∼1.455 eV are seen to originate from the diffused region. A possible explanation for the lack of electrical activity of the Sn is discussed in terms of these results and may lie in the formation of a compensating Sn‐related complex or an accumulation of Zn in the diffused region.