Photoluminescence (PL), high-precision X-ray diffraction, and positron annihilation studies were performed on compressively strained GaIn(N)As/GaAs quantum-well heterostructures in attempts to elucidate optical and structural effects originating from incorporation of nitrogen into the crystal lattice and from post-growth thermal annealing. It was found that annealing left the properties of the nitrogen-free samples rather unchanged, but strongly enhanced PL and blue-shifted the spectrum for GaInNAs/GaAs. None of the as-grown samples contained negatively charged or neutral vacancies within the sensitivity limits of the positron experiments. However, positron traps (likely Ga vacancies) appeared in the annealed GaInNAs/GaAs sample. These defects did not deteriorate strong light emission from annealed GaInNAs/GaAs. (c) 2006 Elsevier B.V. All rights reserved.