The growth of thin InGaN epilayers on GaN by metal-organic vapour-phase epitaxy has been investigated by atomic force microscopy. The effect of increasing the NH3 flow rate on films growing in a 2D growth mode has been examined, and a transition from 2D island nucleation to step-flow growth with increasing NH3 flow tentatively attributed to a decrease in the barrier to adatom incorporation at step edges. Furthermore, increasing the overall reactor pressure, and hence the NH3 partial pressure, has been shown to encourage the formation of 3D nanostructures. This transition from 2D to 3D growth is linked to surface kinetic or thermodynamic effects rather than a change in the composition of the film. (C) 2004 Elsevier B.V. All rights reserved.