Extended X-ray absorption fine structure (EXAFS) above the In K-edge of luminescent InGaN heterostructures provides a unique probe of local structure on an atomic length scale. Through a process of fitting the experimental spectrum, we can refine a model of the probable configuration (the atom type, co-ordination number and radial separation) of the first few shells of neighbouring atoms in the vicinity of a selected probe atom. We present here, for the first time, the In K-edge EXAFS spectrum of an ultrathin, uncapped 'quantum box' (QB) sample and compare it to results we have obtained previously on thick luminescent InGaN epilayers with a range of composition. While the epilayers resemble simple alloys, more or less, the QB sample reveals itself to consist of a two-phase mixture of InN and dilute InGaN alloy. The use of EXAFS to distinguish the local In environments of different InGaN-based heterostructures is likely to provide key information to unlock the puzzle of the origin of luminescence in these important commercial semiconductors. (C) 2002 Elsevier Science B.V. All rights reserved.