Monodisperse iron oxide/microporous silica core/shell composite nanoparticles, core(gamma-Fe2O3)/shell(SiO2), with a diameter of approximately 100 nm and a high magnetization are synthesized by combining sol-gel chemistry and supercritical fluid technology. This one-step processing method, which is easily scalable, allows quick fabrication of materials with controlled properties and in high yield. The particles have a specific magnetic moment (per kg of iron) comparable to that of the bulk maghemite and show superparamagnetic behavior at room temperature. The nanocomposites are proven to be useful as T-2 MRI imaging agent. They also have potential to be used in NMR proximity sensing, theranostic drug delivery, and bioseparation.