The LixMn(2)O(4) to lambda -MnO2 phase transition, as lithium is extracted electrochemically from the spinel structure, has been studied by in situ neutron diffraction. The 'single-phase' composition around 4.1 V is found to be strongly dependent on potential, with the Li content (x) varying between 0.65(9) and 0.49(12). The amount of Li in the cubic lambda -MnO2 phase is 0.27(13) with cell parameter a = 8.115(1) Angstrom, in good agreement with earlier ex situ neutron diffraction results. A simple in situ neutron diffraction method is presented for following structural changes during electrochemical cycling of a lithium insertion compound.