Visible electroluminescence (EL) characteristics of porous Si formed on p, n, p(+)-n, and n(+)-p junction substrates are studied under pulse current regime. We provide experimental proof that porous Si structures characterized by highest EL intensity have simultaneously highest photoluminescence (PL) intensity and lowest surface roughness. PL and EL can be correlated via surface roughness Two types of EL instabilities are observed : fast, with time constant in the milliseconds range and slow, with time constant in the hours range. The fast EL instability is affected by the type of ambient and/or sample temperature during electrical excitation and is likely to be associated with the charging of the porous network.