Spherical gold nanoparticles are synthesized in worm-like catanionic reverse micelles, demonstrating the absence of shape templating except when the growth of the particles is slow. In situ time resolved SAXS measurements show that the microstructure of the microemulsion is conserved during the whole reaction. The liquid–liquid phase transition of the catanionic microemulsion induced by a gentle cooling is used to recover and purify the nanoparticles in a simple manner.