The mechanisms for lithium diffusion in Li x Co0.5Ni0.5O2 were investigated using the galvanostatic intermittent titration technique (GITT). Membrane electrodes prepared with poly(vinylidene fluoride) and carbon black were employed in this study. The measured Brunauer–Emmett–Teller (BET) area of the Li x Co0.5Ni0.5O2 powder was combined with the GITT data to obtain the lithium chemical diffusion coefficient ( D ̌ Li + ), the lithium self-diffusion coefficient (D Li+ ) and the thermodynamic factor (Φ) as a function of Li concentration (x). All three parameters vary non-monotonically with x. A minimum in D ̌ Li + and D Li+ at x=0.5, along with structural changes, suggests the formation of a lithium superlattice at that concentration. The behavior of D ̌ Li + is complex but for x<0.34 it eventually undergoes a continuous decrease due to the metallic character of Li x Co0.5Ni0.5O2. We attribute the limitation of the specific reversible capacity of Li x Co0.5Ni0.5O2 to this decrease in D ̌ Li + and to elevated electrode voltages. Li transport in Li x Co0.5Ni0.5O2 is analyzed taking the variations in the cell parameters and the oxidation states of the Ni, Co and O ions into account.