Using a synchrotron radiation source, extended X-Ray Absorption Fine Structure (EXAFS) measurements were performed at the gallium and indium K-edges of a range of (In,Ga)N samples, grown either by MBE or MOCVD. The resulting determinations of local alloy structure, complemented in selected cases by asymmetric x-ray diffraction reciprocal space mapping (XRD-RSM), show an inequality in the “mixed cation” separations, Ga-In and In-Ga, for samples with InN content less than about 50%. This asymmetry, which increases with decreasing InN content of the layers, is related to the high luminescence efficiency of the materials through a combination of percolation and localisation of excitation on the GaN and InN sub-lattices, respectively.