The composition of lithium manganese spinel Li1+xMn2-xO4 (LMO) cathode materials for lithium-ion batteries is very sensitive with respect to temperature conditions during processing. Elevated calcination temperatures promote the formation of Li2MnO3 secondary phase in addition to the spinel phase. Under heat treatments at lower temperature secondary-phase Li2MnO3 can react with the LMO spinel main phase and form a new spinel phase with higher Li-content. This solid state reaction has been observed but its kinetic behavior has not been investigated. An experimental approach to monitor the change of Li2MnO3 amount during heat treatment on lithium manganese spinel materials is addressed by implementing Electron Paramagnetic Resonance (EPR) spectroscopy. It is shown that for materials prepared initially at 1073 K, the reaction occurs from 673 K to 973 K and it is active between interface of spinel and Li2MnO3. The Li2MnO3 amounts after varied heating temperatures and holding times were quantified and analyzed. The reaction kinetics, based on a quantitative analysis of the EPR resonances, are discussed by using pseudofirst-order and second-order rate laws. Detailed data analysis indicates that the reaction follows different kinetics depending on the microstructure of Li2MnO3.