Migration enhanced epitaxy (MEE) has been generally accepted as an important low-temperature epitaxial growth technique. The main advantage of MEE over conventional molecular beam epitaxy (MBE) according to the accepted model is the enhanced migration of group III elements during growth as proposed by its inventor. Although there have been a number of reflection high-energy electron diffraction (RHEED) and optical studies of the MEE process, certain aspects of the mechanism are not well understood. Using the in-situ techniques of RHEED and reflectance anisotropy (RA), the response of a GaAs(100) surface to the deposition of Ga and to the alternate deposition of Ga and As2 or As4 has been studied. Whilst the well-known 4 x X (X = 2 or 6 ... ) Ga-stabilised surface is obtainable after deposition of Ga (or during the Ga deposition phase of an MEE cycle) at high temperatures (e.g., 580-degrees-C), such a surface is highly disordered at lower temperatures ( < 530-degrees-C) and only obtainable at slow deposition rates at temperatures below 350-degrees-C. After a complete MEE cycle, the surface can only return to the 2 x 4 As-stabilised reconstruction if the temperature is 500-degrees-C or above. Therefore barriers must be present which prevent a well ordered surface being obtained at the end of each Ga and As deposition phase. The RA response to the MEE process is quite independent of temperature, but the RHEED specular beam intensity variation indicates that during the As deposition phase of an MEE cycle the surface morphology is under the control of As and changes in a way similar to MBE.Migration enhanced epitaxy (MEE) has been generally accepted as an important low-temperature epitaxial growth technique. The main advantage of MEE over conventional molecular beam epitaxy (MBE) according to the accepted model is the enhanced migration of group III elements during growth as proposed by its inventor. Although there have been a number of reflection high-energy electron diffraction (RHEED) and optical studies of the MEE process, certain aspects of the mechanism are not well understood. Using the in-situ techniques of RHEED and reflectance anisotropy (RA), the response of a GaAs(100) surface to the deposition of Ga and to the alternate deposition of Ga and As2 or As4 has been studied. Whilst the well-known 4 x X (X = 2 or 6 ... ) Ga-stabilised surface is obtainable after deposition of Ga (or during the Ga deposition phase of an MEE cycle) at high temperatures (e.g., 580-degrees-C), such a surface is highly disordered at lower temperatures ( < 530-degrees-C) and only obtainable at slow deposition rates at temperatures below 350-degrees-C. After a complete MEE cycle, the surface can only return to the 2 x 4 As-stabilised reconstruction if the temperature is 500-degrees-C or above. Therefore barriers must be present which prevent a well ordered surface being obtained at the end of each Ga and As deposition phase. The RA response to the MEE process is quite independent of temperature, but the RHEED specular beam intensity variation indicates that during the As deposition phase of an MEE cycle the surface morphology is under the control of As and changes in a way similar to MBE.