The lead dioxide active mass of positive lead-acid battery plates is a gel-crystal system with proton and electron conductivity of the hydrated gel zones. This paper discusses the influence of Sn 2+ , Sb 3+ , Co 2+ , Mg 2+ and Al 3+ ions, added to the formation electrolyte, upon the stoichiometry, structure and phase composition of the PbO 2 positive active material (PAM) of lead-acid batteries. PAM samples doped with the above metal ions are characterized by: X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and chemical analysis. The obtained results show that different metal ions are incorporated in different quantities in the PbO 2 particles. Under the influence of dopants, the stoichiometric coefficient of lead dioxide decreases, i.e. dopants increase the non-stoichiometry of PbO 2 . The foreign ions in the formation electrolyte exert strong influence on the microstructure of PAM and change the proportion between crystal and hydrated gel zones in the particles.