EN
Thermal expansion of Lu_2 Fe_{17-x}Si_x solid solutions was measured by X-ray powder diffraction. The magnetic ordering in all compounds within the homogeneity range (x≤3. 4) is accompanied by a large spontaneous volume magnetostriction distributed anisotropically over the principal axes of the hexagonal crystal structure. The volume effect ω_s in the ground state reaches 14.7×10^{-3} in Lu_2Fe_{17} and decreases monotonously to 8.9×10^{-3} at x = 3.4 following the reduction of magnetic moment. Despite still large ω_s, the Invar behavior observed in Lu_2Fe_{17} changes to a positive thermal expansion for x >1 due to increasing Curie temperature.