An in situ X-ray absorption fine structure (XAFS) experiment has been performed to observe the evolution of gold nanoparticles in the ionic liquid [BMIM][AuCl₄], by hard X-ray irradiation. The ionic liquid acts as both a reducing agent and a protective ligand. A synchrotron-based X-ray plays the role of the irradiation source, which induces the reduction of the gold species, as well as being a real time probe for XAFS measurements. From the extended X-ray absorption fine structure (EXAFS) fitting results for a series of spectra of gold L₃-edge, it can be seen clearly that there is a single Au–Cl bond breaking process before the formation of Au–Au bonds, which is different from previous reports on the formation of Au nanoparticles by several chemical methods.