Dependence of oxygen deficiency of BaBiO3-delta on temperature and oxygen partial pressure at a state of equilibrium was investigated by thermogravimetry. Two phases, one with 0.00 less than or equal to delta < 0.03 (phase I) and the other with 0.43 < delta less than or equal to 0.50 (phase II), and the first order phase transition between them was observed below 700 degrees C. Besides the two phases, another phase (phase III) with 0.10 < delta < 0.20, which could be regarded as stable at high temperatures, appeared at 750 degrees C, It was observed that the phase transitions of I double left right arrow III and II double left right arrow III are also first order. By increasing the temperature up to 800 degrees C, the delta range of phase III increased to 0.06 < delta < 0.25. The eutectoid point, at which the three phases coexisted, was observed at a temperature of similar to 710 degrees C and an oxygen partial pressure of similar to 10(-3) atm in the relationship between the temperature and the oxygen partial pressure at the two phase region. From the relationship, variations of enthalpy of phase transitions were calculated to be 170 +/- 35 kJ mol(-1), 190 +/- 20 kJ mol(-1) and 80 +/- 5 kJ mol(-1) for I --> II, I --> III and III --> II, respectively.