A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1-xN/GaN(0<x<1) heterostructures deposited on c-plane sapphire substrates by low pressure organometalic vapor phase epitaxy. Motivated by a method suggested by Bowen and Tanner for separating the contributions of strain and composition in cubic crystals, we extended the technique to the case of hexagonal crystals by first principles derivation from elastic strain theory. The technique was evaluated using double-axis and triple-axis reciprocal space maps of 200 and 30 nm AlxGa1-xN layers. The procedure did not require absolute lattice parameter measurements and relied instead on relative measurements of the layer and substrate peak positions. Symmetric and asymmetric reflections of the film and substrate were measured in the double-axis configuration with ω–2θ scans. From the peak separation, the strained lattice parameters were determined. Assuming biaxial strain and linear variation of the relaxed lattice parameter with alloy composition, an equation was derived for composition in terms of the strained lattice parameters. The relaxation distribution in the AlGaN was found to be bimodal. Films with mole fraction of Al greater than 0.4 were completely relaxed and thoroughly cracked, while films with mole fraction of Al less than 0.4 exhibited compositionally dependent relaxation about the fully strained state.