The role of diffusion of charge carriers in radiative recombination processes for porous silicon is discussed. The conducting porous material is modelled on a lattice in which each occupied site represents an Si crystal of nanometre size. To describe the recombination dynamics of particles and holes, two extreme situations are considered: one in which the electron and the hole diffuse as a single entity (exciton-like motion) and a second one in which they diffuse independently of each other (two-species motion). In both cases the recombination is excitonic and takes place on a single site. The present results are compared with other theoretical approaches, originally developed for describing photoluminescence in hydrogenated amorphous silicon (a-Si:H).