The high-precision coulometric titration measurements were used in order to find the temperature and oxygen pressure dependent variations of oxygen homogeneous fields in CaBaFe4O7+δ1 (CBF7) and CaBaFe4O8–δ2 (CBF8) ferrites and to demonstrate a miscibility gap separating bordering compositions CaBaFe4O7.1 and CaBaFe4O7.8. The spinodal mechanism of the structural transition between CBF7 and CBF8 is proposed to be responsible for the excellent mechanical robustness and unusually large thermal expansion coefficient (156∙10−6 К−1 at 400 – 600 °C) of ceramic samples formed during the heating and oxidation of the CBF7 phase in air.