The heterojunction between amorphous silicon (sub)oxides (a-SiOx:H, 0 < x < 2) and crystalline silicon (c-Si) is investigated. We combine chemical vapor deposition with in-system photoelectron spectroscopy in order to determine the valence band offset ΔEV and the interface defect density, being technologically important junction parameters. ΔEV increases from ≈0.3 eV for the a-Si:H/c-Si interface to >4 eV for the a-SiO2/c-Si interface, while the electronic quality of the heterointerface deteriorates. High-bandgap a-SiOx:H is therefore unsuitable for the hole contact in heterojunction solar cells, due to electronic transport hindrance resulting from the large ΔEV. Our method is readily applicable to other heterojunctions.