The post-growth thermal annealing of GaInNAs quantum well (QW) based lasers has been widely employed to optimise the desired laser emission wavelength and efficiency although the mechanism for this process is uncertain. A number of factors have been implicated including the inter-diffusion of gallium and indium atoms; reduction of non-radiative centres; and the enhanced incorporation of nitrogen atoms into the GaInNAs lattice structure. This paper investigates the distribution of indium within as-grown and annealed GaInNAs QW's characterised by medium energy ion scattering. Comparison of the channelled and randomly scattered Ga and In atomic distributions indicates the aggregation of indium in the as-grown QW's occurs on the nanoscale. After annealing at temperatures up to 600 degreesC the indium concentration in the <100> blocking dips is decreased and the strain in the QW is marginally increased. At 640 degreesC surface degradation is observed in the sample and the GaInNAs QW decomposes. (C) 2002 Elsevier Science Ltd. All rights reserved.