Optical and electrical properties of Hg acceptor in GaAs were systematically investigated as a function of Hg concentration. Samples were prepared by high‐energy ion implantation of Hg+into GaAs grown by liquid encapsulated Czochralski method. Annealing was made by rapid thermal annealing using infrared flash lamp at 950 °C during 3 s. Photoluminescence measurements at 2 K revealed that in addition to the well‐defined conduction band to Hg acceptors transition, the mercury‐related so‐called ‘‘g’’ neutral acceptor bound exciton band is found shifted from the carbon‐related ‘‘g’’ line by 0.8 meV. Moreover, two shallow emissions, are formed for net hole concentration ‖NA−ND‖ greater than 2×1017cm−3and 1×1019cm−3, respectively. It is demonstrated that even a typical moderately deep acceptor Hg in GaAs, having ground‐state ionization energy of 52 meV can make multiple shallow emission levels presumably attributed to acceptor–acceptor pairs. ©1995 American Institute of Physics.