We present a comparison between the predictions of two theoretical models and experimental results on ultrathin GaAs layers with a thickness in the range from 1 to 8 ML embedded in bulk (AlxGa1-x)As, 0.30 less than or equal to x less than or equal to 0.34. The theoretical predictions, obtained from an empirical tight-binding Green's-function approach and from the usual effective-mass approximation, are compared to photoluminescence and photoluminescence excitation data of a set of multiple quantum-well samples grown by metal-organic vapor-phase epitaxy on substrates with different misorientations. We find that the optical transitions are narrower in samples with slightly misoriented substrates, although their spectral position remains unchanged. This suggests that the substrate misorientation favors a good quality of the ternary alloy. The observed optical transitions of our thin layers compare well with the predictions of both models. However, the lack of a reliable exciton theory for such ultrathin layers becomes apparent.