In a previous investigation, a diamagnetic band of the magnetic circular dichroism of the optical absorption (MCDA) in semi-insulating GaAs centered at 1.19 eV was observed and attributed to the neutral diamagnetic charge state of EL2,EL20.In this letter, we show a direct correlation between the signal intensities of two diamagnetic MCDA bands located at 0.93 and 1.19 eV and the concentration of neutral diamagneticEL20centers in different GaAs samples. A calibration factor is presented for the absolute MCDA intensities. This opens up the possibility of measuring the concentrations ofEL20andEL2+using the same technique. ©1997 American Institute of Physics.