Near-lattice-matched GaN/AlInN quantum wells are investigated by means of contactless electroreflectance (CER) and temperature-dependent photoluminescence (PL). Large Stokes shifts, up to 400 meV, between PL peak energies and CER resonances are identified. This Stokes shift is attributed to large potential profile fluctuations (PPFs) in the AlInN barriers. Further evidence for such PPFs and for the additional influence of QW width fluctuations is provided by temperature-dependent PL measurements, demonstrating large PL halfwidths and clear "S-shape" behavior. The influence of a large Stokes shift on the correct determination of the value of the built-in electric field is discussed, and it is shown that PL measurements may lead to a significant overestimation of the built-in electric field in GaN/AlInN QWs. (c) 2008 American Institute of Physics.