The wavelength shifts in the photoluminescence (PL) from low indium composition (similar to 3%) InGaN epitaxial thin films, grown on sapphire substrates by metalorganic chemical vapour deposition, has been studied by a combination of experiment and theory. As temperature increases from 6 K, the PL peak energy red-shifts very slightly first, then blue-shifts to reach a maximum at near 100 K, and red-shifts again till room temperature. This unique PL behaviour, indicating the existence of the phase separation, is interpreted qualitatively from the spatial variation of band structure due to the In-compositional fluctuation. Theoretical calculation, based upon a model involving the band-tail states in the radiative recombination, explains the experimental data successfully. (c) 2005 Elsevier B.V. All rights reserved.