The key issues of localised electronic states in determining the behaviour of porous silicon (PS) devices is highlighted. We discuss recombination parameters of this material to be calculated starting from the study of electrical features of metal-PS-crystalline silicon heterostructures. The analysis of spectral and transient curves of the photocurrent and the thermostimulated current allows identifying several electronic states with an energy of 0.045, 0.07, 0.14 eV and concentration in the range of 0.7 x 10(12) -2.4 x 10(13) cm(-3). It is shown, that at low temperature the capture cross-section of the nonequilibrium charge carriers can attain a huge value up to 1-8 x 10(-12) cm(2), that is caused by the formation of large-scale aggregation of structural defects or levels in the silicon nanocrystals. The possible origin of such defects and mechanism of multiple electron capture by traps are discussed. (C) 2002 Published by Elsevier Science Ltd.