We analyze the dependence of the interface defect density Dit in amorphous/crystalline (α-Si:H/c-Si) heterostructures on the microscopic properties of the ultrathin (10nm) undoped α-Si:H films. It is shown that the hydrogen bonding configuration, probed by infrared spectroscopy, determines the α-Si:H network disorder, which in turn governs the annealing behavior of these structures upon a short thermal treatment at moderate temperatures (T ≤ 200°C). While the as-deposited Dit seems to be determined by the local structure at the interface, the final state of the annealed samples is determined by the bulk α-Si:H network strain as reflected in the valence band tail slope. Thus it appears valid to treat the equilibrated α-Si:H/c-Si interface as a 2D-layer that has the projected defect properties of the 3D α-Si:H bulk. © 2010 Materials Research Society.