A p(2 x 2)surface reconstruction is observed over the 800-900C range for StrTiO3, the temperature range which is used in the oxygen anneal of Y, Ba, Cu layers on SrTiO3(100) substrates in the synthesis of high Tc superconducting thin films. As observed by Augers electron spectroscopy, high-temperature annealing causes calcium to segregate to the surface. Initially, at a Ca content of 3% upon a SrO-.terminated SrTiO3(100) surface, the Ca atom is incorporated in a superstructure, which is attributed to multiple scattering across the two topmost interfaces. When the surface Ca content is below 1% the p(2 x 2) surface reconstruction is induced. The p(2 x 2) surface reconstruction disappears when the calcium content is removed.