LiMn2O4 and LiMn2-xCoxO4 (0.1 less than or equal to x less than or equal to 0.4) samples were prepared by a precipitation technique followed by an appropriate heat treatment in the 600-800 degrees C temperature range, The spinel materials were characterized by powder X-ray diffraction and electrochemical measurements. The differential capacity versus voltage plots indicate that the presence of cobalt strongly influences the order/disorder phase transition of the inserted lithium ions in the upper potential region of the spinel phase, This can be proven by X-ray diffraction measurements of delithiated spinels, showing two coexisting spinel phases in the undoped system whereas a single phase behavior in the cobalt-doped spinel system is observed. These materials show excellent capacity retention during extended cycling.