Results of thermal characterisation based on the phase lag of photoacoustic signal for front-rear surface illumination of porous silicon and nitrided porous silicon membranes for gas sensing devices are presented. Thermal conductivity values in good agreement with literature values have been obtained, confirming the usefulness and reliability of photothermal methods in the investigation of new materials for sensors and microsystems. Preliminary results of stokes-antistokes Raman investigations are also reported.