Photoluminescence and photoluminescence excitation spectroscopies, scanning electron microscopy, and atomic force microscopy were used to study the photoluminescence mechanism in porous silicon. The dependences of photoluminescence parameters on electrochemical etching regimes, excitation light wavelength, and vacuum ageing have been investigated. We show that intensive and broad "red" luminescence band (similar to 600-800 nm) is non-elementary, and can be decomposed into three elementary bands. The mechanisms of the elementary bands are discussed.