Non-conventional GaAs/GaAlAs structures, with an AlAs monolayer inserted at different positions in wells, have been investigated to observe the behaviour of excitons in quantum wells. The energy and the oscillator strength for different transitions are calculated as a function of the probe position. The differential reflectivity of the spectrum for some samples is measured in order to test our theoretical simulations. We confirmed the observation of certain parity-forbidden transitions only in the most asymmetric samples.