We investigated the effect of thermal annealing on the low temperature excitonic reflectivity (LTER) spectra of a high quality GaAs/ AlGaAs single quantum well (QW) grown by molecular beam epitaxy. Collecting LTER spectra at 2 K, on different samples annealed at a constant temperature of 950 °C for different time durations, we could resolve three different series of confined transitions up to a final value of the diffusion length L D = 66 A ̊ . These transitions correspond with the (E 1-HH 1), (E 10LH 1) and (E 2-HH 2) electron-hole states respectively. Depending on the transition investigated and the degree of intermixing, these transitions exhibit different confinement energies which allow the QW profile to be probed in great detail. This establishes, on a purely experimental basis, LTER as one of the most powerful (and non-destructive) ways to monitor the effect of QW intermixing (QWI) when processing GaAs/AlGaAs photonic integrated circuits. Next, solving the Schrödinger equation to determine the position of all confined states in the OW as a function of the annealing conditions, we could predict the change in the transition energies expected for the localized states as a function of L D . Comparing with our experimental values, we find that, provided the nominal QW thickness (100 Å) has been adjusted slightly to a final value of about 110 Å, there is very satisfactory agreement (for the five annealed samples) between the theoretical predictions and the experimental results. To the best of our knowledge, this is the only all-optical evidence that, in this range of investigation, Fick's second law can be applied satisfactorily and that it is not necessary to call for any concentration-dependent mechanism to explain the effect of QWI on both the fundamental and excited states.