Photoluminescence (PL) and decay time measurements have been carried out in the energy range from 1.3 eV to 2.3 eV changing the temperature from room temperature (RT) to 10 K on different series of porous silicon samples, coming from lightly (series A) and heavily doped (series B, C) p-type Si substrates. The PL from A samples exhibits a sharp rise and decay, with a maximum at 100 K, while from the B and C series the PL intensity, after an initial increase from RT to 150 K, remains constant for T going from 100 K to 10 K. The temperature dependence of the radiative lifetime in p(-) samples is explained by invoking confined, localized excitons in nanometre-sized structures. For p(+) samples the PL lifetime is shorter at low T so that a mon efficient radiative process occurs. We discuss this effect in terms of the probability for finding boron impurities even in nanometre-sized structures. The role of these impurities in the excitonic recombination processes is recalled as a possible explanation of the PL behaviour.