The dependence of oxygen deficiency, delta, of BaBi1-xPbxO3-delta on temperaure T, and oxygen partial pressure, P-O2’ was investigated by thermogravimetry. A linear relationship with proportionality constant about -1/2 between log delta and log pO(2) observed for the specimens with x = 0.25, 0.5 and 0.75 suggested that effective concentration of electron carrier was independent of delta. The decrease of electrical conductivity with delta could be attributed to the decrease of electron mobility. From our thermogravimetry data, deduced were the decrease of valence state of Bi as delta increased and the localization of electrons generated from oxygen vacancy around Bi site, which agreed with the band diagrams which was proposed based on the results of photoelectron spectroscopy and X-ray absorption.