Oxygen K shell X-ray emission spectroscopy has been used to demonstrate the covalent involvement of shallow core 5d electrons in the bonding in HgO. The extent of core hybridisation with the O-2p levels is much more pronounced in HgO than in ZnO or CdO and shows an inverse correlation with the atomic binding energy of the shallow core state. Band structure calculations confirm the importance of mixing between Hg-5d and O-2p states in HgO. However, the assumption that direct intra-atomic mixing between Hg-6s and Hg-5d orbitals determines the linear stereochemistry in HgO is shown to be incorrect. (C) 2004 Elsevier B.V. All rights reserved.