Carbon nanotubes (CNTs) have been successfully coated with a covalently bonded polymerbrush of negatively charged poly(3-sulfopropylamino methacrylate) (PSPM) by in situpolymerization employing atomic transfer radical polymerization (ATRP) from initiatingsilanes attached to the CNTs before the polymerization. The CNT-bonded brush forms apolymer layer or shell-like structure around the CNTs and provides colloidal stabilizationfor the CNTs in aqueous media. In situ syntheses of nanocrystalline CdS and magnetic ironoxide in the polymer brushes lead to the formation of hybrid nanocompositesconsisting of nanoparticle-containing PSPM-coated CNTs that remain readilydispersible and stable in aqueous media. The hybrid nanostructures are synthesized byion exchange with the cations of the sulfonate groups of the PSPM followed byprecipitation and were followed by stepwise zeta potential measurements andTEM. Such structures could have applications in the design of more complexstructures and devices. The general synthetic scheme can be extended to include othernanoparticles as brush cargo to broaden the utility or functionality of the CNTs.TEM data shows nanocrystalline CdS in the range of 5–8 nm embedded in thePSPM brush and nanocrystalline iron oxide with a size between 2 and 4 nm, withthe former consistent with UV–vis spectroscopy and fluorescence measurements.