We show that the yield of single-walled carbon nanotubes (SWCNTs) grown with bimetallic catalysts is a strong function of their atomic-scale composition. A series of composition-ally-tuned Ni x Fe 1Àx bimetallic catalysts with a constant mean diameter of 2.0 nm are used to catalyze the growth of nanotubes via a floating catalyst method. Increasing the Fe con-tent in the catalysts is found to lower the fraction of SWCNTs in the collected as-grown product. Based on a simple surface-to-volume model, these results are explained by the higher carbon solubility of Fe compared to Ni which results in a larger amount of carbon precipitation and the formation of multi-walled tubes when the nanotubes are nucleated from catalysts with high Fe content. Overall, our study demonstrates that the size and com-position of bimetallic catalysts must be precisely controlled to obtain high yields of SWCNTs for large-scale production.