A phase diagram for LixCoO2 was determined by potentiometric and calorimetric measurements as a function of lithium concentration (x) in the host electrode. These measurements show clear evidence for a reversible phase transition between hexagonal and monoclinic structures near x = 0.5 in LixCoO2 during electrochemical-calorimetric cycling at various cell temperatures. The phase transition and the range over which it occurs were found to be temperature dependent. It was well correlated with observations that, during discharge of lithium-ion cells having LixCoO2 as positive electrode material, the overall exothermic heat effect contains a transient contribution between exothermic and endothermic heat generation near x = 0.5.