The effect of mild oxidation in air of LiFe1-yMnyPO4 (0 <= y <= 1) powders is studied via in situ and ex situ X-ray diffraction, thermogravimetric analysis and Mossbauer spectroscopy. Structural modifications involve an anisotropic change in the lattice parameters upon heat-treatment with conservation of the olivine-type structure up to similar to 500 degrees C. The extrusion of part of iron in the form of alpha-Fe2O3 is observed, which results in the oxidation of iron in the structure and the creation of vacancies, while Mn remains stable at a +2 oxidation state. Mass gain and unit-cell volume variations upon heat-treatment under air are found to be proportional to y in the pristine composition. In addition, the electrochemical signature of powders annealed at similar to 300 degrees C significantly differs from the stoichiometdc ones. Two solid solution contributions arise from the Fe3+/Fe2+ redox couple, and the Mn3+/Mn2+ reaction upon Li+ electrochemical insertion/extraction is reported for the first time as a solid solution reaction. (C) 2013 The Electrochemical Society.