The equilibrium conductance of LaAlO3/SrTiO3 (LAO/STO)-heterointerfaces was investigated at high temperatures (950¿K-1100¿K) as a function of ambient oxygen partial pressure (pO2). Metallic LAO/STO-interfaces were obtained for LAO grown on STO single crystals as well as on STO-buffered (La,Sr)(Al,Ta)O3 substrates. For both structures, the high temperature sheet carrier density nS of the LAO/STO-interface saturates at a value of about 1¿×¿1014¿cm¿2 for reducing conditions, which indicates the presence of interfacial donor states. A significant decrease of nS is observed at high oxygen partial pressures. According to the defect chemistry model of donor-doped STO, this behavior for oxidizing conditions can be attributed to the formation of Sr-vacancies as charge compensating defects.