In order to overcome the capacity fading of LiMn2O4 in lithium batteries, various substitutions for Mn have been proposed. The structural implications of substitution in LiMn2-xMxO4 with x = 0.5, i.e. with exactly 1/4 octahedral (16d-site) cations replaced, are investigated here. For this stoichiometry, cationic ordering was known previously for M = Mg and Zn, resulting in a superstructure with primitive cubic symmetry. Given the poor chemical contrast of X-ray diffraction between Mn and Co, Ni or Cu, LiMn1.5M0.5O4 samples were studied by neutron diffraction and IR spectroscopy. Both techniques show the occurence of cationic ordering for M = Ni and Cu, but not for Co or Ga. In the case of M = Zn, further complication due Li/Zn ordering on the tetrahedral (8a) site is well resolved by FTIR. This investigation shows that the main driving force for octahedral cation ordering is the charge difference between Mn and M atoms.