We have performed density functional calculations on tetragonal SnO and PbO (litharge) in the space group P4/nmm with the specific intention of examining the rôle played by Sn 5s and Pb 6s lone pairs in stabilizing the structure, and in giving rise to semi-metallic behavior (of SnO at ambient pressure and of PbO in the γ phase). Use of the electron localization function has permitted real-space visualization of the lone pair in these structures. We also discuss the electronic structure of the orthorhombic PbO (massicot, space group Pbma) which again has localized lone pairs, contrary to some earlier expectation. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.