Communication: Chemical control over the size of nanoparticles is a prerequisite for their application. The synthesis of protective polymers having cyano or mercapto groups (e.g., MMS-NVP, see Figure) is reported, together with the influence of these polymers on the size and size distribution of gold nanoparticles synthesized by KBH4 reduction. The results obtained using a linear protective polymer without a cyano or mercapto group are compared.