Porous Si Fabry-Perot films are coated with Pd via immersion plating. The materials are characterized by electron microscopy and infrared spectroscopy. The Pd-coated porous Si samples exhibit distinct Fabry-Perot fringes in the optical reflection spectrum due to thin film optical interference in the porous Si layer, though the reflectivity spectrum loses fidelity upon Pd coating. The effect of H-2 exposure on the interference spectrum is studied. Absorption of hydrogen into Pd induces a lattice expansion, which results in a shift of the optical fringes and a decrease in the reflected intensity. The detection limit measured at room temperature is similar to0.2% (by volume) in an N-2 carrier gas, with a response time of a few seconds.