We study by synchrotron-based time-resolved small angle X-ray scattering the growth of silver shells on gold nanorods in the presence of hexadecyltrimethylammo-nium chloride (CTAC) as a function of the silver/gold molar ratio K. For all but the highest K values, the silver precursor is completely converted to the solid phase. The growth rate of the width decreases exponentially, and its starting value is linear in K, with a slope of 10.3 ± 0.2 nm/s/M. A simple growth model including these features accounts for the magnitude of the evolution time. Assuming that permeation across the CTAC bilayer is the rate-limiting step we obtain a permeability of 1 µm/s.