Changes in lattice parameter in high‐purity polycrystalline (2Hδ) aluminum nitride due to oxygen contamination have been examined by convergent beam electron diffraction. The operative lattice distortion was determined by computer simulation to be a continuous increase inc/aratio commensurate with constant volume as impurity concentration increases and a fine break in hexagonal symmetry. These results are consistent with the distortions associated with planar polytypoid structures at higher oxygen leve