An investigation was conducted in order to characterize the softening and hardening behavior of binary iron alloys at temperatures of 77 – 411 K. These temperatures correspond to a homologous temperature range of 0.043 – 0.227 T m (where T m is the melting temperature of Fe) and coincide with the temperature range over which alloy softening is generally observed in body-centered cubic metals. A total of nineteen binary Fe alloy systems were investigated. Alloys were prepared by the arc-melting of high-purity Fe and high-purity solute additions. The primary means of evaluation involved hardness testing, which provides an expedient means of determining the mechanical behavior of such a large number of alloys. Results showed that the atomic radius ratio of solute-to-Fe was the primary factor in controlling low-temperature hardness of the binary Fe alloys. Alloy softening observed in fifteen of the alloy systems was attributed to an intrinsic mechanism, believed to be the lowering of the Peierls (lattice friction) stress. The fifteen alloy systems which exhibited alloy softening similar to softening observed in other b.c.c. systems included those with titanium, vanadium, chromium, molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, and platinum. Both softening and hardening rates could be correlated with soluteto-Fe atomic radius ratios. The atomic radius ratios of the fifteen solutes all lie within the ±15 percent size limit considered favorable for solid solution formation in Fe. The remaining four solute additions lie outside this favorable-size zone. Alloy softening was observed at very low solute concentrations with niobium and tantalum additions, and is attributed to an extrinsic mechanism, probably associated with scavenging of interstitial impurities. Alloy softening did not occur in binary iron-zirconium and ironhafnium alloy systems, probably because of the very large solute-to-iron atomic radius ratios in these systems.