Process of interconversion of tetragonal Sr_0.8Gd_0.2CoO_3-d (with ordered Sr²⁺/Gd³⁺ cations and anion vacancies) to cubic one (with disordered  structure) was studied by X-ray structural  and thermal analysis at 1100-1473 K. It was shown that the transformation is a first order diffuse phase transition. The ramp rate does not affect cation and anion vacancies disordering which is controlled by thermodynamic parameters of the processes in the solid whereas cubic to tetragonal transition is kinetically controlled. A theory of diffuse phase transition was applied to quantitatively analyze heat capacity temperature dependence