Abstract In x Al 1− x N is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In x Al 1- x N has not been researched extensively. In this study, thin In x Al 1− x N epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al y Ga 1− y N layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In x Al 1− x N epilayers grown on GaN and compressively strained epilayers grown on Al y Ga 1− y N, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In x Al 1− x N layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.