This project is devoted to the defect chemistry of LiFePO4, doped LiFePO4, FePO4 as well as the solid solutions Li1-yFePO4 and LixFePO4. Effects of donor or acceptor doping on electronic and ionic conductivity are studied, the partial pressure dependence and the change of the transport properties within the solid solution range. Conductivity data are obtained by impedance spectroscopy as well as DC (direct current) polarization measurements using ion or electron blocking electrodes. From the data the major defects as well as defect reactions can be derived and a defect model can be established. Moreover, the combination of the defect model and the experimental data allows the calculation of defect migration and reaction enthalpies.