The metal complex formation and the electrical properties of amorphous solid polymer electrolytes, based on poly(butadiene–acrylonitrile) copolymer (PBAN) and CoCl 2 , have been studied over the homogeneity region of the system limited by the CoCl 2 concentration of 1.89molkg −1 . It has been found that ionic conductivity is carried out by the unipolar anion transfer at lower CoCl 2 concentrations (up to 0.10molkg −1 ). As the CoCl 2 concentration increases, electronic conductivity appears in addition to ionic conductivity, and the former becomes dominant, starting from 0.38molkg −1 . It has been shown that the nature of charge carriers is determined by the composition of metal complexes formed by CoCl 2 and the macromolecular solvent PBAN. At lower concentrations, the [Co 2 L 2 Cl 4 ] 0 dimers are the predominant species (L being macromolecule side groups CN), and their dissociation is followed by the formation of mobile Cl − anions and immobile binuclear [Co 2 Cl 3 ] + complexes. As CoCl 2 concentration increases, polynuclear [Co n L 2 Cl 2n ] 0 (n>2) complexes appear (L being CN and CC groups of PBAN). Specific features of chemical bonds in π-complexes of transition metals result in the appearance of electronic charge carriers. The abrupt increase in conductivity observed at the highest CoCl 2 concentration is connected with the formation of a percolation network of polynuclear [Co n L 2 Cl 2n ] 0 complexes.