The protonic species formed in spinel lambda-MnO2 by proton exchange for lithium ions in Li1+xMn2-xO4 precursors have been characterised by infrared and inelastic neutron scattering spectroscopies. Proton diffusion into the lattice appears to depend on the presence of octahedral manganese vacancies, with formation of lattice hydroxyl co-existing with water. Mechanisms of proton transfer are considered in relation to lithium-proton exchange during reinsertion of lithium into the materials, and for insertion of additional protons by chemical reduction.