Gold nanorods were synthesized by the colloidal seed-mediated, surfactant-assisted approach [Gou et at., Chem. Mater. 2005,17,3668-3672] using CTAB (hexadecylcetyltrimethylammonium bromide) obtained from ten different suppliers. The yield of gold nanorods depended strongly on the CTAB used: with the same recipe, three of the CTABs did not yield nanorods and produced only spherical gold particles, whereas the other CTABs yielded nanorods with nearly 100% yield. These results suggest that an impurity in the CTAB is very important for nanorod formation.