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.