Radio frequency plasma-assisted molecular beam epitaxy (MBE) growth of GaN on InSb (100) has been investigated. This combination is interesting because a 45 degrees rotation of a cubic epitaxial GaN laver could result in a nearly "lattice-matched" system. The growth of low-temperature buffer layers and initial substrate nitridation at 275 degrees C on the morphology of the subsequent growth at 450 degrees C were considered. Nitridation produced a smooth, mixed InN and Sb-N layer, whilst annealing to 450 degrees C resulted in the loss of the Sb nitride component and disruption of the InN, causing exposure of the underlying substrate and surface roughening. Similarly thin buffer layers (similar to 8 Angstrom) were found to crystallise and island at 450 degrees C but allowed substrate damage. By contrast, thicker buffer lavers (similar to 80 Angstrom) remained smooth and continuous and protected the substrate but did not crystallise. Subsequent growth morphologies reflected the surface quality of the underlying layers, however all layers were polycrystalline wurtzite GaN and no evidence was found for crystalline cubic GaN formation.