Nanoparticles play an important role in chemical and biological sciences due to their ability to bind and concentrate many molecules on their surface(1). Polymers(1,2) and silica(3-7) are widely used to make nanoparticles, but efforts to make nanoparticles from borosilicate glass-which exhibits high tolerance to chemicals and solvents, combined with excellent mechanical and thermal stability(8-10)-have proved unsuccessful. Here we show that borosilicate nanoparticles (100-500 nm in size) can be synthesized by simply mixing a silicon-boron binary oxide solution, prepared using non-aqueous organic solvents, with water. This induces a vigorous exothermic phase separation in which borosilicate nanoparticles burst out of a silica phase. In addition to potential applications in the life sciences, monodisperse borosilicate particles could also have applications in the production of photonic bandgap devices with high optical contrast, contrast agents for ultrasonic microscopy or chemical filtration membranes(11-13).