Recent reinvestigations of the crystal chemistry of Li4SiO4-based solid solutions Li(4+x)M(x)Si(1-x)O(4) (M = B, Al) are presented in this paper. The starting lithium salts used (LiOH . H2O or Li2CO3) and the atmosphere (air, argon) in which the thermal treatments are carried out, play a major role in impurity formation. The influence of Li2CO3 and/or remaining unreacted LiOH on the cation transport properties of Li4SiO4-based materials is discussed.