We report a method of directly synthesizing gold nanoparticles that are coated with poly(ethylene glycol) (PEG) for applications in biology and medicine. The particles are grown within the cores of micelle-like structures formed by pH-responsive branched copolymers of poly(ethylene glycol) methacrylate (PEGMA) and 2-diethylamino methacrylate (DEAMA). The process is cheap and scalable, with PEGylated gold particles prepared in two straightforward steps from commercially available monomers and salts. The formation of gold nanoparticles with mean size of 45 Å is confirmed by transmission electron microscopy. Kinetic studies of nanoparticle growth by small-angle X-ray scattering (SAXS) show that the particles form slowly over a period of >17 hours. This process can be modified by exposure to high-intensity X-rays, which cause widespread rapid nucleation leading to a larger number of smaller particles.