We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe90Ni10, Fe85Al4Si11, Ni99Fe1 and Ni90Fe10 powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T < 0.4 T-m) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall-Petch relationship. We explain this abnormal Hall-Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall-Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing similar to 1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing similar to 1.0 at.% impurities). Published by Elsevier Ltd on behalf of Acta Materialia Inc.