We present results from kinetic studies on the surface-initiated atom transfer radical polymerization in the preparation of polymer brush-coated magnetic particles from a heterogeneous system. It is shown that a controlled reaction behavior and a reproducible surface functionalization with end-tethered polymers are achieved, although the reaction advances gradually from a biphasic solid-liquid mixture to a stable colloidal dispersion of the nanoobjects. Although the initiator-functional magnetite nanoparticles initially form a precipitate, the formation of a polymer layer on the particle surface in the course of the reaction contributes to a sterical stabilization in dispersion. We thoroughly investigated the development of the initial heterogeneous system with time and in various concentration regimes by simultaneously monitoring the monomer conversion, molar mass, the hydrodynamic diameter of the nanoobjects, and the magnetite content of the dispersions at different reaction times. The results indicate first-order chain growth kinetics with respect to the monomer and narrow molar mass distributions, demonstrating good control on the particle architecture. (C) 2009 Wiley Periodicals, Inc J Polym Sci Part A: Polym Chem 47: 7012-7020, 2009