A multiple quantum‐well structure grown by organometallic chemical vapor deposition and exhibiting a thickness gradient over the sample surface has been analyzed by spectroscopic and spatially resolved ellipsometry. The sample has been scanned in energy from 1.4 to 4.0 eV, with 5–10‐meV resolution, and in position over a 46‐mm line, with a 100‐&mgr;m optical resolution. Using multilayer modeling we have first determined the structural parameters and particularly the aluminum concentration in the barrier, and the barrier and the quantum‐well thicknesses. These two thicknesses vary from 95 to 10 A˚ along the 46‐mm scanned line, while their ratio as well as the aluminum concentration (64%) remain constant. The ellipsometric spectra, namely, the effective dielectric function which can be deduced from the tan &PSgr; and cos &Dgr; curves, allow for the determination of the multiple quantum‐well optical transitions around &Ggr;. In the thicker part of the wafer the optical spectra exhibit the well‐known feature of a multiple quantum well associated toN=1, 2, and 3 heavy holes → electron transitions. As the thicknesses decrease, the coupling between quantum well increases, and the structure becomes a superlattice. For a barrier thickness of 30 A˚, we observe the splitting of the fundamental level into two components: the first attributed to the symmetrical wave function and the second to the antisymmetrical wave function. The splitting is observed for both the heavy‐ and light‐hole transitions. As the coupling between wells still increases, the dielectric function of the superlattice tends towards the one of the GaAlAs alloy with an average aluminum concentration of 32%. The evolution of the optical transitions versus barrier and quantum‐well thickness has also been investigated theoretically by solving the Schro¨dinger equation for a periodic structure. The calculations have been done for two values of two conduction‐band offsets: 60% and 85%. The overall agreement between theory and experiment is very good for the 60% conduction‐band offset.