A full understanding of the mechanism of the formation of a two-dimensional electron gas (2DEG) at the interface between insulating LaAlO₃ (LAO) thin films and bulk SrTiO₃ (STO) crystals is a prerequisite for the full exploitation of this class of materials. Here, by using a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements, it is shown that a structural and electronic reconstruction of the interface occurs before the realization of the 2DEG.