Аннотация:Investigation results are presented for the temperature dependence of lattice parameters of LaxCe1-xMnSi system alloys. The anomaly of thermal expansion behaviour which has been revealed in the ranges of 140-180 K and 280-320 K is explained as a result of electronic structure rearrangement in cerium sublattice. The results obtained testify to the fact that structural and magnetic transformations in the alloys studied are sensitive to the processes responsible for Ce variable valency.