High-quality GaAs/(Al,Ga)As heterostructures with quantum wells grown by molecular-beam-epitaxy technology are experimentally studied by means of optical spectroscopy of exciton states. The exciton resonances observed in the reflectance spectra are analyzed in the framework of phenomenological and microscopic models. The exciton energies, the radiative (â.,Γ0) and nonradiative (â.,Γ) broadening, and the phases of resonant reflection are obtained from the modeling for each exciton resonance. These parameters are used for careful analysis of the parameters and quality of the structures. Particular attention is paid to the exciton energies and phases, which are used to determine the quantum-well and barrier-layer thicknesses with high accuracy.