Infrared and Raman spectroscopy have been used to investigate the Li-ion battery cathode materials Li x (Mn y Fe1−y )PO4 for 0< x ≤1 and y =0.0, 0.5 and 1.0. In the Raman spectrum of Li(Mn0.5Fe0.5)PO4, the frequency of the P O 4 3 − symmetric stretching mode is very similar to that found in LiFePO4 and LiMnPO4, suggesting that the P–O bond lengths are similar across the entire Li(Mn y Fe1−y )PO4 solid-solution series. The more localized ν1 and ν3 bands (symmetric and antisymmetric P O 4 3 − stretching modes, respectively) in the infrared spectra exhibit two-mode behavior for all three Li(Mn y Fe1−y )PO4 compositions studied, whereas the P O 4 3 − antisymmetric bending modes (ν4) and the Li+ cage mode frequencies vary with Mn2+ concentration. The vibrational modes in Li x (Mn0.5Fe0.5)PO4 are sensitive to lithium content (x) and are, thus, used to investigate changes in the (Mn0.5Fe0.5)PO4 framework under cycling. The lack of spectral changes during the Fe2+/Fe3+ redox couple (3.7V versus Li+/Li) suggests that the P O 4 3 − anions have similar local environments in Li(Mn0.5 (II)Fe0.5 (II))PO4 and Li0.5(Mn0.5 (II)Fe0.5 (III))PO4. Two-phase behavior is confirmed during the Mn2+/Mn3+ redox couple (4.2V), and the infrared spectrum for (Mn0.5 (III)Fe0.5 (III))PO4 is similar to that of FePO4 since both phases contain P O 4 3 − anions coordinated only to trivalent transition metal ions. However, the P O 4 3 − anions are much more distorted in (Mn0.5 (III)Fe0.5 (III))PO4 compared to FePO4, probably as a result of deformation of the MnO6 octahedra induced by Jahn–Teller active Mn3+ ions in the (Mn0.5 (III)Fe0.5 (III))PO4 structure. Raman spectra suggest that the carbon layer coating the Li(Mn0.5Fe0.5)PO4 particles (to improve electronic contact between particles) is predominantly composed of sp2-hybridized carbon atoms.