Magnetic susceptibility of iron—tin alloys containing 0.45, 0.82, 1.33, 1.83, and 2.14 at.% tin have been studied as a function of temperature up to 1500°K. Tin influences the magnetic properties of iron in an unusual way. The mass magnetic susceptibility of &ggr; iron is increased by about 1% due to 0.45 at.% tin. The paramagnetic moments of the bcc phase, determined from the susceptibility data between 1300° and 1500°K, show a small increase with increasing tin content. These results are, in principle, consistent with the ferromagnetic properties studied by Fallot (1936) indicating that tin does not act as simple diluent in regard to the magnetic behavior of iron. Electrical resistivities of iron—tin alloys, determined at 4.2°, 77.2°, and 297.0°K, show that tin increases the resistivity of iron very considerably, i.e., about 8 &mgr;&OHgr; cm per 1 at.% at 4.2°K. Lattice parameters of iron—tin solid solutions up to 5 at.% tin (quenched from 1013°K) have been measured at 298°K. None of the present theories are capable of explaining satisfactorily the observed behavior.