The structural and optical properties of a series of GaN/Al0.18Ga0.82N multiple quantum well (MQW) Structures, in which the well thickness was varied between 2 and 8 nm, grown on a-plane (1 1 (2) over bar 0) GaN on r-plane (1 (1) over bar 0 2) sapphire Substrates have been investigated. High-resolution X-ray diffraction and low-angle X-ray reflectivity measurements were used to determine the well and barrier thicknesses and the barrier composition after matrix transformation of the (binary) elastic constants for the appropriate coordinates, and assuming a pseudo-morphically strained system. The microstructure of the (1 1 (2) over bar 0) samples is dominated by l(1)-type basal-plane stacking faults (BSF) terminated by partial dislocations or prismatic stacking faults, as determined by conventional and high-resolution transmission electron microscopy. The low temperature photoluminescence (PL) spectra of the QW structures show two emission bands which are assigned (partly based on photoluminescence excitation (PLE) spectroscopy) to excitons that are confined solely by the quantum wells and, at lower energy, those carriers that recombine in the region where the wells are intersected by BSFs. Both bands shift to higher energy with decreasing quantum well thickness. The optical data indicate that the non-polar (1 1 (2) over bar 0) GaN/AlGaN system is free of polarization-induced electric fields, since the QW exciton emission energy is not below the band-edge emission energy of the GaN template. (C) 2008 Elsevier B.V. All rights reserved.