The synthesis of core-shell particles with a poly(epsilon-caprolactone) (PCL) shell and magnetite (Fe3O4) contents of between 10 wt.-% and 41 wt.-% proceeds by surface-initiated ring-opening polymerization of epsilon-caprolactone to give surface-immobilized oligomers with (M) over bar (n) between 1 400 g(.)mol(-1) and 11 500 g(.)mol(-1). The particles are dispersable in good solvents for the PCL shell. Magnetization experiments on the resulting superparamagnetic ferrofluids give a core-size distribution with an average diameter, d(v), of about 9.7 nm.