Layered Li-x(Mn1-yCoy)O-2 can deliver a discharge capacity on cycling of 200 mA h g(-1) at 0.1 mA cm(-2) Structural transformations occurring in these solid solutions during electrochemical cycling of lithium have been investigated by combining the results from powder neutron diffraction with those from cyclic voltammetry and incremental capacity measurements. The layered structure of the solid solutions converts to spinel on cycling. The rate of conversion depends on the cobalt content. There is evidence that conversion to spinel commences even within the first cycle of the solid solution with y = 0.1 whereas compounds with the composition y = 0.3 show almost no conversion to spinel within three cycles and only some conversion after 30 cycles. A two-phase model composed of layered and cubic spinels does not significantly improve the fit to the neutron data for the y = 0.1 composition within the first cycle indicating that there is little evidence of long range order in any spinel that is formed. Clear diffraction evidence of an ordered spinel phase for y = 0.1 is observed after four cycles. Conversion to spinel does not appear to have a deleterious effect on the capacity retention on cycling.