Near‐infrared optical absorption in undoped bulk GaAs ingots is shown to be due essentially to the presence of the main deep donor EL2. Measurement of the corresponding absorption represents the first known method of quantitative determination of that level in semi‐insulating material. Furthermore, complete quenching of the corresponding absorption is shown to occur under high intensity illumination. This strong effect, reported for the first time, can be directly related to the existence of a metastable state for the level EL2 presenting a strong lattice relaxation.