We report a comprehensive characterization of the microstructure of non-polar (1 1 (2) over bar 0) a-plane AlxGa1-xN epilayers on (1 (1) over bar 0 2) r-plane sapphire substrates over the entire Al composition range using high-resolution X-ray diffraction. All a-plane AlGaN epilayers show an anisotropic in-plane mosaicity which is strongly influenced by Al incorporation and growth conditions. We use data from symmetric and skew-symmetric reflections combined with Williamson-Hall analysis to estimate the mosaic parameters and follow trends across the Al composition. We also analyze the dependence of the microstructure on thickness for a set of Al0.5Ga0.5N epilayers. (C) 2010 Elsevier B.V. All rights reserved.