Several researchers have proposed that band discontinuities at semiconductor heterojunctions may be "tuned" by inserting very thin layers of foreign atoms at the interface which an thought to induce an "interface dipole." Modifications of the apparent valence-band offset, as measured by photoelectron spectroscopy (PES)I have been indeed observed upon Si insertion at GaAs-AlAs interfaces. and they have been generally interpreted as real band-offset changes. However, there is an alternative explanation of the photoemission results in terms of band-bending effects. Here, we present results of PES experiments designed to test the two opposing interpretations. We have examined the effect of Si insertion at polar (100) and nonpolar (110) interfaces, and we have studied the insertion of Si (n-type) and Be (p-type) intralayers. Similar results are obtained for polar and nonpolar interfaces, and effects of opposite sign are observed for Si and Be intralayers. These results can be readily interpreted in terms of a band-bending profile modification upon Si or Be insertion. Additional PES experiments performed at different substrate temperatures have allowed us to test the proposed band profiles. From the surface photovoltage effects induced at low temperature, we obtain evidence for sample band bending which is consistent with the room-temperature band profiles proposed. Hence, our results can be completely understood within a "band-bending interpretation," calling into question the interpretation in terms of a "band-offset tuning effect:"