Non-contact atomic force microscopy (NC-AFM) has been used to image MgO(1 0 0) cleaved in situ under ultrahigh vacuum. The image quality was found to be dependent on the quality of the cleave, with the best images revealing an atomically flat surface with rectangular depressions one atomic layer deep. Other features observed include a line defect attributable to missing rows of Mg2+ and O-2(-) ions, bright features consistent with adatoms and cleave-induced damage structure. Images of rougher surfaces contained a high concentration of 'up-down' steps. These latter images are similar to tapping mode images of MgO(1 0 0) cleaved in air. (C) 2003 Elsevier Science B.V. All rights reserved.