The phase transformation behavior of a single-phase tetragonal Sr0.8Gd0.2CoO3-delta(with ordered distribution of Sr2+/Gd3+ cations and anion vacancies) was investigated by TG-DSC and XRD at 1100-1473 K and oxygen partial pressure p(O-2) from 1 to 5.10(4) Pa. The first-order smeared order-disorder (o-d) phase transition involving heat absorption was observed at about 1383 K under O-2-Ar flow with p(O-2) > 2.5 kPa. The crystal structure of the high-temperature phase was found to be cubic perovskite with disordered Sr2+/Gd3+ cations and anion vacancies. The temperature of o-d transition at p(O-2) > 2.5 kPa was not influenced by the heating rate or oxygen partial pressure. It was shown that at p(O-2) < 2.5 kPa the o-d transition gives rise to Sr0,8Gd0.2CoO3-delta decomposition to form CoO and Sr0.8Gd0.2CoO3-delta - Ruddlesden-Popper type phase with novel composition. The phase boundaries between the ordered tetragonal Sr0.8Gd0.2CoO3-delta, disordered cubic Sr0.8Gd0.2CoO3-delta and CoO + Sr0.8Gd0.2CoO3-delta composite as a function of temperature and p(O-2) values were determined.