The paper gives the results of X-ray measurements on the variation of the lattice parameter of lithium with temperature. The mean thermal coefficient of expansion of lithium varies from 3.5 × 10^-5 at -914°C to 4.7 × 10^-5 at 20°C. The expansion of the material over this range of temperature is closely represented by the equation a_t = a₀(1 + αt + βt² + γt³) where a₀ = 3.4992kx; α = 4.6640 × 10^-5; β = 3.8791 × 10^-8; γ = -11.445 × 10^-11. After deformation at liquid air temperature lithium is found to contain a face-centred cubic structure as well as a body centred cubic structure, which confirms the findings of C. S. Barrett. The lattice parameters of these structures at -194°C were found to be 3.4760kx (body-centred cubic) and 4.370kx (face-centred cubic). After annealing the material at room temperature and afterwards obtaining its x-ray pattern at liquid air temperature the body-centred cubic structure remained alone.