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Hydrogen is known to passivate Si dangling bonds at the Si− SiO 2 interface, but the subsequent arrival of H+ at the interface causes depassivation of Si-H bonds. Here we report first-principles density functional calculations, showing that, contrary to conventional assumptions, depassivation is not a two-step process, namely, neutralization of H+ by a Si electron and subsequent formation of an H 2 molecule. Instead, we establish that H+ is the only stable charge state at the interface and that H+ reacts directly with Si-H, forming an H 2 molecule and a positively charged dangling bond (P b center). As a result, H-induced interface-trap formation does not depend on the availability of Si electrons.