The composition of InxGa1-xN nanorods grown by molecular beam epitaxy with nominal x=0.5 has been mapped by electron microscopy using Z-contrast imaging and x-ray microanalysis. This shows a coherent and highly strained core-shell structure with a near-atomically sharp boundary between a Ga-rich shell (x∼0.3) and an In-rich core (x∼0.7), which itself has In- and Ga-rich platelets alternating along the growth axis. It is proposed that the shell and core regions are lateral and vertical growth sectors, with the core structure determined by spinodal decomposition. © 2014 IOP Publishing Ltd.