Irreversible thermal reconstruction of vacuum-annealed MnO(100)−(1 x 1) to (2 x 2) and (6 x 6) structures was observed with low-energy electron diffraction with transition temperatures of 800 and 1000K, respectively. The relatively weak (2 x 2) structure did not always precede (6 x 6) reconstruction and in particular was inhibited by the presence of small amounts of calcium impurities. The reconstructions occurred with no detectable change in stoichiometry as measured by Auger electron spectroscopy and X-ray photo-electron spectroscopy. The data were in general agreement with rumpling of surface oxygen as predicted by several theoretical studies of ionic crystal lattice stability. Thermally Induced Surface Reconstruction of MnO(100). M.A.Langell, N.R.Cameron: Surface Science, 1987, 185[1-2], 105-19