Janus nanoparticles have been synthesized consisting of 5 or 10 nm magnetite nanoparticles coated on one side with a pH-dependent and temperature-independent polymer (polyacrylic acid, PAA), and on the other side functionalized by a second (tail) polymer that is either pH-independent polymer (polystyrene sodium sulfonate, PSSNa, or polydimethylamino ethylmethacrylate, PDMAEMA) or temperature-dependent polymer (poly(N-isopropyl acrylamide), PNIPAM). The Janus Nanoparticles are obtained by grafting the second polymer to the exposed surfaces of PAA-coated magnetite nanoparticles adsorbed onto positively charged silica beads. Individually dispersed Janus nanoparticles were obtained by repulsion from the beads on reversal of the silica surface charge when the solution pH was increased. These Janus nanoparticles are dispersed stably as individual particles at high pH values and low temperatures, but can self assemble at low pH values (PSSNa and PDMAEMA) or at high temperatures (>31°C) (PNIPAM) to form stable dispersions of clusters of approximately 35-100 nm in hydrodynamic diameter, depending on grafter polymer type and molecular weight. The addition of small amounts of PAA-coated magnetic nanoparticles to the Janus nanoparticle suspension at low pH values resulted in a further increase in the final cluster size of pH responsive Janus nanoparticles. The Janus nanoparticle compositions were verified using FTIR and XPS, and their structures observed directly by TEM. Their clustering behavior is analyzed by dynamic light scattering, Zeta potential measurements and Cryo-Microscopy. Monte Carlo simulation results compared favorably with experimental observations and showed the formation of small, elongated clusters similar in structure to those observed in cryo-TEM images.