The reduction mechanism of oxidizing agents at silicon and porous silicon electrodes has been investigated in relation to light emission from the porous semiconductor. Oxidizing agents with a positive redox potential are shown to inject holes into HF-pretreated silicon. However, as the degree of oxide formation on the electrode surface increases, reduction by electron capture from the conduction band becomes important. Holes can be injected from H2O2 when the intermediate OH· radical is formed chemically in solution; electrochemical reduction of H2O2 generally does not involve the valence band. Hole injection at the open-circuit potential from certain oxidizing agents at porous silicon electrodes results in visible luminescence with characteristics similar to those of the emission observed during anodic oxidation in indifferent electrolyte. A mechanism similar to that proposed for anodic luminescence is suggested.