Abstract Paramagnetic and X-ray studies have been made on iron-rich iron-germanium alloys. The lower boundaries of the γ-loop have been determined from magnetic susceptibility measurements. The γ-loop extends to about 5·5 w o germanium. At high temperatures the paramagnetic behavior of iron-germanium solid solutions follows the Curie-Weiss law. These measurements show that germanium in solution in iron produces quantitatively different behavior in Curie point, Bohr magneton number, and lattice parameter than does silicon. Simple theoretical descriptions based on the Heisenberg model do not correctly describe the paramagnetism of binary iron alloys. The lattice spacings of solid solutions of germanium in iron, quenched from about 600°C, have been determined accurately as a function of atomic percentage of germanium. The plot of lattice constant vs. composition is a straight line to 10·45 a o germanium. The abrupt change to zero in the slope of this line establishes the α-phase boundary at that composition, at approximately 600°C. A new scheme is proposed for relating lattice parameter changes to atomic volume of the solute. Under the proposed assumptions, a linear relation of lattice parameter to atomic volumes of solvent and solute is found. They also serve equally well as a basis for the lattice dilation of the nickel lattice with germanium as the solute. Furthermore, the changes in volume per atom as a function of composition for the systems iron-germanium and nickel-germanium agree very well up to 5 a o germanium. The behavior of germanium as a solute in iron and nickel is in contrast to its behavior in the group 1B metals where the effect of valence difference is marked.