Nanoindentation hardness measurements were performed on nanocrystalline (nc-) Cu alloys to test recent molecular dynamics predictions that (i) solute segregation to grain boundaries can lead to significant strengthening and (ii) solutes with large size mismatch with Cu are most effective. Results show that the hardness of nc-Cu90Nb10 is greater than 5 GPa, more than double that of pure nc-Cu, whereas similar additions of Fe solute have nearly no effect. These results are in good agreement with simulations. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.