Polymer–magnetite nanoparticle complexes that respond to both magnetic fields and to temperature have been demonstrated. Novel alkyl halide-functional bis(diethylphosphonate) esters were prepared and utilized as initiators for polymerizing N-isopropylacrylamide by controlled atom transfer radical polymerization. The phosphonate esters were removed after polymerization to afford poly(N-isopropylacrylamide) with a bis(phosphonic acid) moiety precisely placed at one terminus. The bis(phosphonic acid) endgroups were adsorbed onto magnetite nanoparticles to yield nanoscale complexes that were stable against any polymer desorption and that were colloidally-stable in physiological media. Thus, the bis(phosphonate) endgroup provides a robust anchoring moiety onto the magnetite. Hydrodynamic sizes of the complexes were predicted with a density distribution model and using the measured sizes of the magnetite cores. Good agreement between the measured and predicted hydrodynamic sizes suggested that the complexes were primarily discrete, non-agglomerated nanoparticles. The complexes exhibited thermosensitive aggregation behavior near the lower critical solution temperature of the poly(N-isopropylacrylamide) component.