We determine the band offsets in amorphous/crystalline silicon [a-Si:H/c-Si{111}] heterojunctions using combined data from photoelectron spectroscopy and surface photovoltage measurements on structures comprising a-Si:H layers with device-relevant thickness (10nm). Altering the a-Si:H hydrogen (H) content CH by the choice of deposition conditions, we observe a systematic retreat of the a-Si:H valence band edge leading to an increase of the band gap and the valence band offset ΔEV with CH by about 13meV/at.H. The discrepancy with the 30-40meV/at.H predicted by theory can be consistently explained by the compensating effect of enhanced topological disorder imposed by the increasing density of microvoids as revealed by an analysis of the H microstructure. Thus we highlight the necessity of explicitly including the details of the H configuration in a theoretical treatment of the a-Si:H/c-Si heterojunction. © 2011 American Physical Society.