We performed first-principles calculations to study the stability of different Sr adatom structures that have been proposed to explain some of the observed reconstructions of the (001) surface of strontium titanate (Kubo and Nozoye 2003 Surf. Sci. 542 177). From surface free energy calculations, we constructed a phase diagram indicating the range of environmental conditions over which each surface reconstruction is stable. We compared these results with Kubo and Nozoye’s experimental observations, and concluded that low Sr adatom coverage structures can only be explained if the surface is far from equilibrium. Intermediate coverage structures are stable only if the surface is in or very nearly in equilibrium with the strontium oxide.