The electronic structure of the abrupt GaAs–Ga1−x Al x As (001) heterojunction is compared with that of the heterojunction in which the transition from GaAs to GaAs–Ga1−x Al x As occurs over several atomic layers across the interface. The effects of varying the width of the compositionally graded region on interfacial boundary conditions and on quantum well energy levels are studied using the tight‐binding method. The compositional grading is modeled in the virtual crystal approximation. Substantial shifts in quantum well energies are found. For example, the ground state energy for a quantum well with 15 central GaAs layers and bounded by AlAs increases by 30% for graded region widths of six alloy layers. The effects of grading on the electronic wave function at the interface are studied by expanding it in terms of bulk states with complex wave vector values. The relative contribution of the bulk states from the neighborhood of the Γ and X points to typical conduction band states are determined. It is found that grading the interface over a small number of layers (two or three) effectively eliminates the X‐point state contribution. Thus, the effective mass approximation, which includes Γ‐point related bulk states only, is shown to be better justified for slightly graded interfaces.