We applied a photoacoustic (PA) technique to study the optical absorption and thermal diffusivity of porous silicon (Psi) samples. The Psi layers were formed on p-type Si wafers in an HF electrolyte. Three kinds of Psi samples that had undergone chemical post-etching were studied before and after UV irradiation for 2 h. We observed that a strong confinement effect occurred in all of the Psi samples from the blue shift of the band gap energy compared with that of crystalline Si, while the effective thermal diffusivities were almost two orders of the magnitude smaller than that of conventional Si crystals. The band gap shifted to higher energy and the effective thermal diffusivity decreased as the post-etching time increased. In the case of a Psi sample that was not chemically post-etched, the optical absorption and effective thermal diffusivity before and after UV irradiation were almost unchanged. However, for the Psi samples that were chemically post-etched, the optical absorption decreased and the effective thermal diffusivity increased after UV irradiation. This indicates that Psi samples that are post-etched are more readily affected by UV irradiation, i.e., oxidized by replacing Si-H, bonds by Si-O-x, than those that do not undergo post-etching. (c) 2005 Elsevier B.V. All rights reserved.