The potential-composition (phi -x) curve of a stoichiometric LiMn2O4 spinel shows an anomaly that a new voltage step appears at x=0.7 (in LixMn2O4) at low temperature (T less than or equal to 278 K). To investigate the relation between this anomaly and structural transition due to the Jahn-Teller distortion, we recorded the phi -x profiles at 298 and 273 K with several nonstoichiometric specimens, Li-excess Li1+yMn2-yO4 as well as Li-deficient compounds apparently expressed by Li1-yMn2O4, because it is sensitive to stoichiometry whether the compound undergoes the transition or not. It was found that Li-deficient compounds (y' = 0.03-0.05) showed a similar anomalous step, though no first-order transition was observed in the relevant temperature range by the DSC technique. The same thing was also found for a Li-excess sample with y = 0.01. These facts indicate that the origin of the anomaly is not directly related to the Jahn-Teller distortion. (C) 2000 Elsevier Science B.V. All rights reserved.