We use absorption, photoluminescence, and x‐ray diffraction spectra of two GaAs/AlAs type II heterostructures, whose GaAs well thicknesses differ by &bartil;4 A˚ to obtain a direct measurement of the individual quantum confinement energy shifts of the heavy hole, light hole, and electron levels. We find that excitonic absorption linewidths are dominated by inhomogeneous broadening that arises from half‐monolayer well‐thickness fluctuations. For self‐consistency these shifts are applied to separately determine the individual valence‐band and conduction‐band offsets.