Spectral and transient photovoltage (PV) phenomena are investigated in hydrogenated and oxidised free standing mesoporous silicon (meso-PS) layers. The size and the surface conditioning were changed systematically. The absorption tails of hydrogenated and oxidised meso-PS have been related to disorder in the bulk nanoparticle (similar to p+-Si) and to a disordered surface region. The limits of surface and diffusion PV are discussed. For as prepared meso-PS, a fast PV response has been observed. The fast component disappeared and the PV transients became more and more retarded in time with ongoing decrease of the mean dimensions of the Si nanoparticles. The retarded component in the PV transients has been assigned to interparticle transport in ondulated quantum wires and correlates with the dielectric relaxation time.