Chemical and mechanical stability of the Ba-added high-Tc phase has been studied using XRD and DTA for samples of Bi_0.9Pb_0.2Sr_1.0Ba_xCa_1.0Cu_1.6O_z with x=0 and 0.05. Some impurities were observed in all samples after the heat treatment at 300-820°C. BaPb_0.75Bi_0.25O₃ was also formed in the Ba-added samples heated at 700-800°C. These impurities were thought to be formed from the amorphous phase which was produced by mechanical grinding of the high-Tc phase. The impurities disappeared and an almost monophasic sample of the high-Tc phase was obtained by heating at 850°C. The lattice parameter c of the Ba-added high-Tc phase was longer than that of the Ba-free one above 800°C. The parameter c of the Ba-added sample heated at 850°C increased with an increase of heating time and was saturated over 48h. The increase of c was due to the Ba dissolution in the high-Tc phase.