Open circuit voltage (OCV) was measured as a function of temperature and state of charge (SOC) for six kinds of lithium ion cells. The following cells were used: four kinds of commercial cell using a LiCoO2 cathode and a graphite or hard carbon anode; a trial manufacture cell using a Li-Ni-Co complex oxide cathode and a graphite-coke hybrid carbon anode; and a trial manufacture cell using a LiMn2O4 cathode and a graphite anode. The entropy change in the cell reaction was determined by calculating the derivative of the OCV with temperature. Results were compared and discussed to determine the influence of the phase transition in the electrode materials due to cell reaction. It was clarified that the entropy change in cells using a LiCoO2 cathode is negative except for the part of the SOC region where LixCoO2 phase transition occurred. An endothermic reaction then occurs during discharge and an exothermic reaction during charge. In cells using LiCoO2 cathodes, there was a fluctuation in the entropy change originating from the LixCoO2 phase transition in the SOC range between 70% and 90%. This fluctuation was influenced by temperature and by additives or excess lithium in the cathode material. The entropy change in both cells using a Li-Ni-Co complex oxide cathode or a LiMn2O4 cathode was comparatively small.