Wurtzitic nitride quantum wells grown along the (0001) axis experience a large Stark effect induced by the differences of spontaneous and piezoelectric polarizations between the well and barrier materials. In AlxGa1-xN/GaN quantum wells, due to the adverse actions of quantum confinement, that blue-shifts transition energies, and of the Stark field, that red-shifts them, the transition energies are nearly independent of barrier compositions at a particular well thickness (L-0 similar to 2.6 nm), at least for x less than or equal to 0.3. The effect of alloy fluctuations is then minimal, as reflected by a minimum in the quantum well luminescence linewidth when L similar to L-0 for wells grown by molecular beam epitaxy on silicon or sapphire substrates. We use this effect to estimate the average variances of well widths and alloy composition fluctuations. Both results are in good agreement with, respectively, a scanning tunneling microscopy study of GaN (0001) surfaces, and estimates based on the lateral extent of the quantum well excitons. We then discuss the optical properties of the AlxGa1-xN barrier material, with particular emphasis on the symmetry of the valence band maximum (Gamma(9) or Gamma(7)). We show that it may play an important role in the apparent barrier luminescence efficiency. We analyse the possible consequences of the barrier Gamma(9)-Gamma(7) crossover on the AlxGa1-xN/GaN quantum well properties. (C) 2004 Elsevier Ltd. All rights reserved.