The electrochemical oxidation of p-type porous silicon prepared under different conditions is investigated. During the oxidation process, bright visible light is emitted from the surface. The time evolution of integrated electroluminescence signals obtained from a silicon photodiode are shown. The signal duration and relative intensity as a function of process parameters during the formation of the porous silicon and during the anodic oxidation are discussed with a view to establish the origin of the luminescence. The experimental results seem to give more evidence for the quantum confinement model. It does, however, not exclude other theories.