Structural, electrical and electrochemical properties of Mn-substituted phospho-olivines LiFe1−y Mn y PO4 were investigated and compared to those of LiFePO4. Rietvield refined XRD patterns taken in the course of delithiation process showed apparent difference between phase compositions of these cathode materials upon lithium extraction. Contrary to the LiFePO4 and LiMnPO4 compositions for which a two-phase mechanism of electrochemical lithium extraction/insertion is observed, in case of Mn-substituted LiFe1−y Mn y PO4 samples a single-phase mechanism of deintercalation was observed in the studied range of lithium concentration. Electrochemical characterization of the cathode materials were performed in Li/Li+/Li x Fe1−y Mn y PO4-type cells for y =0.0, 0.25, 0.55, 0.75 and 1.0 compositions. Voltammery studies showed low reversibility of the lithium extraction process in the high-voltage “manganese” range, while in the “iron” range the reversibility of lithium extraction is high. Impedance measurements of the LiFe1−y Mn y PO4 cathode materials, which enabled separation of the ionic and electronic components of their entire electrical conductivity, showed distinct influence of Mn content on the electronic part of conductivity. EIS measurements performed at different states of cell charge revealed that the charge-transfer impedance in Li x Fe1−y Mn y PO4 is much lower than that of Li x FePO4.