Syndiotactic 1,2-polybutadiene nanoparticles (volume average diameter 13 nm) were functionalized in aqueous dispersion by free radical mercaptan addition. By appropriate choice of both water-soluble radical initiator and mercaptan concentration, nanoparticles with a degree of functionalization of up to 85% were prepared using 3-mercaptopropionic acid methyl ester and 3-mercaptopropionic acid. Only a minor portion of double bonds formed cycles instead of the desired thiol-ene addition products. The composition and structure of the nanoparticles were elucidated by combination of elemental analysis, NMR, IR, DLS, and TEM. Highly hydrophilic mercaptans (3-mercaptopropanesulfonic acid sodium salts), in contrast. only reacted with surface accessible double bonds to afford stable and redispersible nanoparticles solely stabilized with covalently bound moieties on their surface. Analogous grafting of the tripeptide glutathione was demonstrated.