Semi‐insulating GaAs materials, undoped or doped with concentration of chromium varying from 6×1015to 4×1017cm−3, have been studied using both Hall effect measurements and optical absorption measurements. It is definitively concluded that compensation comes from the presence of the deep donor EL2 in undoped materials, and from both this deep donor and the deep acceptor related to chromium in Cr‐doped materials. Sets of curves are given which allow the determination ofND‐NA, the concentration of shallow donors and acceptors, knowing the Hall mobility and the Cr concentration in a given sample. Such curves can be a working tool for assessing any piece of semi‐insulating GaAs in a routine way.