The total entropy of inserting lithium into Li xCoO 2 was determined from measurements of the temperature-dependence of equilibrated voltages in electrochemical half-cells. Changes in the entropy of the lithiation reaction with composition were as large as 9.0 9.0 k B/atom, and as large as 4.2 k B/atom within the layered trigonal structure of Li xCoO 2. Three contributions to the entropy of lithiation for the trigonal phase were assessed by independent experiment and calculations. The configurational entropy from lithium-vacancy disorder was significant, accounting for more than half of the full entropy of lithiation. The phonon entropy of lithiation was determined from measurements of inelastic neutron scattering, but its changes with lithium concentration were found to be small. Electronic structure calculations show that the electronic entropy of lithiation also makes a contribution.