Silicon nanowires were grown by chemical vapour deposition on gold catalysts using SiH4 and HCl diluted in H2. The effects of HCl on the wires and the catalysts were investigated for various HCl partial pressures. Keeping all other parameters constant, gold migration on the silicon surface is found to be dramatically reduced by the surface chlorination induced by HCl. We then use HCl to control gold migration and show the existence of a 'diffusion-limited minimum diameter'. This diameter limit arises from the surface migration kinetics and it sets a lower bound on the wire diameter distribution.