In contrast to previous belief, we show that oxide is present at the silicon/dilute-fluoride electrolyte interface even near the onset of anodic current, which corresponds to the regime of porous-silicon formation. This point is supported by the observation of a transient anodic current when the potential is maintained in this region until a steady-state current is reached, then stepped to a value near that of the open-circuit potential. In situ infrared investigations confirm the presence of a submonolayer oxide film. Upon polarizing the interface, the electrochemical current and the surface concentrations of oxide and hydrogen exhibit slow relaxations. These transient features are accounted for in a simple kinetic model, which assumes two parallel dissolution pathways and a slow, potential-dependent relaxation of the electrochemically active surface area. In this framework, the transition to the electropolishing regime is found to correspond to unit coverage of the electrochemically active surface by the oxide.