Photoluminescence techniques have been used to detect and characterise the ∼ 1.40 eV emission band in an Mn-doped GaAs MBE epilayer and heated semi-insulating Cr-doped GaAs. An activation energy ∼ 110 ± 5 meV is obtained from the temperature quenching of the emission intensity, and evidence indicated that physical movement of the Mn atom is responsible for type conversion.