Chromium has been observed to be responsible for a band of absorption in semi‐insulating GaAs. The corresponding absorption coefficient has been carefully calibrated using three different chemical analysis methods. It is shown that the spatial distributions of Cr in semi‐insulating GaAs ingots can be obtained very easily using optical absorption measurements and that the segregation coefficient of Cr in GaAs is equal to 8.9×10−4. The nature of the optical transitions due to the presence of Cr is discussed in detail and it is shown that they occur mainly on the Cr3+in semi‐insulating material.