Li((1-x))Mn(0.7)Fec(0.3)PO(4) (0.00 <= x <= 1.0) samples, obtained by electrochemical cycling, were investigated by powder X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The purpose of this work is to map phase transition information for samples at quasi equilibrium i.e. after a long relaxation time. The required high accuracy solid Li+ content was determined by atomic emission using the standard addition method. From XRD analysis of the charging cycle samples, the phase diagram was shown to exhibit a solid solution (0 <= x <= 0.42), followed by a two phases system exhibiting both olivine and heterosite structures (0.42 < x <= similar to 0.91) and finally above x similar to 0.91 the olivine phase is no longer detectable. The same basic pattern was found during the discharge process. Combined analysis of FTIR spectra and XRD data showed a strong correlation, where vibration bands at 690 and 600 cm(-1) could be uniquely attributed to the heterosite structure.