Spinel oxides with compositions LiMn2O4 and Li1.23Mn1.77O4 were studied using an air-tight electrochemical lithium cell mounted on an X-ray powder diffractometer. The combination of electrochemical and X-ray data unambiguously shows the appearance of a tetragonal phase as soon as lithium intercalation in the host oxide begins. The lithium intercalation reaction is found to be two-phase for both oxide hosts, in spite of the high initial manganese oxidation state in Li1.23Mn1.77O4 (+ 3.82), which lies well above the expected Jahn-Teller distortion limit(+ 3.5). The tetragonal distortion is much stronger in lithiated Li2Mn2O4 than in Li1.23+xMn1.77O4.