The remarkable voltage-tunable electroluminescence (VTEL) observed on porous silicon-electrolyte junctions is investigated in relation to material morphology and electrolysis parameters. The electroluminescence (EL) is obtained upon cathodic polarization of n-type porous silicon in contact with aqueous solutions containing the persulphate ion. The observed long-lived EL shows a reversible spectral shift as large as 300 nm for an external bias variation of about 0.6 V. The study of the EL behaviour as a function of the external voltage and the persulphate ion concentration shows that while the amplitude of the EL is proportional to the intensity of the exchanged current, the spectral position is only determined by the applied voltage. A qualitative model, taking into account the voltage dependence of the charge injection probability into the size-distributed silicon crystallites, gives a good description of the observed VTEL behaviour. In a similar manner, cathodic polarization induces a dramatic change in the porous silicon photoluminescence. It leads to a reversible, highly contrasted and energy-selective quenching of the photoluminescence (QPL) for a polarization variation of only about 500 mV. A spectral blue shift, along with a significant narrowing of the PL line accompanies the observed strong QPL. This results from selective quenching starting at the low luminescence energy and reaching progressively the high luminescence energy as the cathodic polarization is increased. Just as for VTEL, this selective character of the QPL can be explained by a voltage-induced enhancement of charge injection into the size-distributed silicon nanocrystallites.