The resistivity of lithium and three LiMg alloys with magnesium concentrations of 1.0, 5.0 and 10 at.% has been measured from 2 to 60K. Below 10K a term proportional to T² and to the residual resistivity is identified and associated with inelastic impurity scattering of electrons. At higher temperatures the deviations from Matthiessen's rule are found to become essentially independent of concentration.