The stability of various possible terminations of the CoS2(1 × 1) surface have been explored and theoretical expectations are found to agree with experiment. With extensive annealing, there is a phase separation at the (100) surface of CoS2. Sulfur segregation to the surface leads to a significant change in the largely sulfur bands due to changes in the hybridized bands, with cobalt. Resonant photoemission spectra indicate clearly that the hybridized cobalt and sulfur bands, characteristic of the CoS 2 bulk, lie at higher binding energies than those of segregatedsulfur layers. This is discussed in terms of the stability of various surface structures.