Single crystal Ba_(0.6)K_(0.4)BiO_3 was studied by using XPS.The results show that there isno evidence for the presence of Bi~(5+) in single crystal of Ba_(0.6)K_(0.4)BiO_3;instead,there is evidence forthe presence of mixed valent Bi~(3+) and Bi~((3+)-δ).Moreover,the O1s XPS results suggest the presenceof mixed valent O~(2-) and O~(1-),and the valence band spectrum suggests the high degree of covalencyfor Bi—O bonds. Single crystal Ba_ (0.6) K_ (0.4) BiO_3was studied by using XPS.The results show that there is no evidence for the presence of Bi ~ (5+) in single crystal of Ba_ (0.6) K_ (0.4) BiO_3; there is evidence of the presence of mixed valent Bi ~ (3+) and Bi ~ ((3 +) - δ) .Moreover, the O1s XPS results suggest the presenceof mixed valentO ~ (2-) andO ~ (1-) , and the valence band spectrum suggests the high degree of covalency for Bi-O bonds.