The effect of oxygen stoichiometry on the transition metal ordering and electrochemical activity of LiMn2−x Ni x O4 solid solutions was investigated. Temperature–oxygen-partial-pressure–composition (pO2–T–x) diagrams of ordered and disordered phases of LiMn2−x Ni x O4 (0.50≥ x ≥0.36) in the vicinity of order–disorder transition temperature (T c) was constructed by means of infrared spectroscopy, thermogravimetric analysis and galvanostatic measurements. Despite their simplicity and limitations over traditional diffraction techniques, all three techniques offered near excellent capability to distinguish ordered and disordered phases. The effect of oxygen-partial-pressure (pO2) in the annealing atmosphere and nickel content of the spinel on T c was studied. The transition temperature increased with pO2 and nickel content, except in oxygen-rich (pO2 =1) atmosphere for the maximum nickel content spinel of LiMn1.5Ni0.5O4. An explanation for the dependence of the transition temperature on the two variables and changes induced by the post-fabrication heat treatments is provided.