We present here a complete set of experimental results, obtained by electron paramagnetic resonance (EPR) and deep-level transient spectroscopy (DLTS), on the so-called EL2 defect in GaAs. It is obtained on semi-insulating materials and specially doped materials grown as semi-insulating ones, which have been submitted to electron irradiation, thermal treatments, and annealing followed by a quench. First, we show that there are two types of defects which give rise to the same EPR spectrum associated with the antisite As Ga: the one associated with EL2, since it presents its well-characterized metastable property, and another one associated with the isolated As Ga, which is not metastable. Second, we demonstrate that an EL2 defect can be transformed into an isolated As Ga by a thermal treatment. Third, we describe how EL2 defects can be regenerated by a low-temperature treatment in materials which have been annealed and quenched. These results, together with considerations on self-diffusion in GaAs, allow us to conclude that EL2 is a complex formed by an isolated As Ga and an intrinsic interstitial defect, namely As i or Ga i. Finally, we studied the kinetics of EL2 regeneration by DLTS in quenched material; since this regeneration occurs through the interstitial mobility and since the associated activation energy is similar to the one found for As i mobility in electron irradiated p-type material, we deduce that EL2 is the complex As Ga+ As i. All these results, as well as the ones provided by the literature, can be understood if the stable state of EL2 corresponds to As i in second-neighbor position of As Ga while the metastable state corresponds to As i in first-neighbor position.