Transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made to investigate the microstructural behaviour of rock-salt CdO layers grown on (0 0 1) GaAs substrates with two different buffer layers of ZnS and ZnO/ZnS. It is shown that the CdO layer grown on the ZnS buffer layer has epitaxial relationship to the GaAs substrate, even though a low density of CdO nano-crystallites (18-25 nm across) are formed at the CdO/ZnS interface region. TEM and TED results show that the CdO layer grown on the ZnO/ZnS buffer layer is polycrystalline. TED results also show that the ZnO buffer layer is polycrystalline with a mixture of wurtzite and zinc-blende phases, which results in the formation of polycrystalline CdO layers. (C) 2003 Published by Elsevier Science B.V.