The influence of an addition of Sn on the physical properties of a Fe&sngbnd;Ni Invar alloy was studied, the addition being designed to improve the mechanical properties of the Invar by precipitation hardening. Mo¨ssbauer effect with Fe57and Sn119, magnetic and dilatometric measurements, as well as hardness tests were applied. It was found that the solubility of Sn at 1130°C is at least 4.73 at.%, and 1.8 at.% at 700°C. Precipitation hardening was achieved by the formation of an intermetallic compound identified as Ni3Sn2. The hardness of the alloy increases, as does the coefficient of thermal expansion with Sn content. To counterbalance the Ni depletion of the matrix due to the formation of the intermetallic compound, a corresponding quantity of Ni has to be added. When this is done, the alloy exhibits an approximately 50% increase in hardness and an optimal coefficient of thermal expansion of 2.6 × 10−6°C−1.