Relationship between variation of Bi and O valence states in perovskite bismuthates Ba_1-xK_xBiO₃ (BKBO), grown in modified synthesis conditions, and their superconducting state properties is presented. Chemical composition, crystal structure and phase purity of BKBO was determined via micro X-ray fluorescence (μXRF), energy-dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction technique (XRD). X-ray photoemission spectroscopy (XPS) and SQUID magnetometry showed variations of electronic structure and superconducting state properties, i.e., lower and upper critical fields, and transition temperature for the samples synthesized at various conditions. We report that, multiple valencies of bismuth, observed already in BKBO systems and assigned to disproportionation of bismuth valence states, lead in fact to deterioration of superconducting state properties.