We have studied the thickness dependent transition from smooth to rough surfaces/interfaces of Ga(As,N)/GaAs MQWs with different nitrogen content. Three criteria were used to determine whether the samples are smooth or rough. RHEED indicates the surface quality of the samples during growth. Ex situ XRD was applied to characterize the superlattice interfaces; AFM images were used to determine the surface morphology. The samples were categorized in rough and smooth according to the criteria mentioned above. These results were compared with critical thicknesses determined by a theoretical model. (C) 2002 Elsevier Science B.V. All rights reserved.