As-anodized porous Si photonic structures experience drifts of their optical properties upon prolonged exposure to air due to spontaneous oxidation of the metastabe Si-H bonds terminating the pores surface. This is a critical limitation in the case of optical coupled optical microcavities, where both long term usage and stable nanometer resolution are required. With a view to stabilizing these structures, the surface of porous Si was chemically modified by hydrosilylation. The resulting surfaces are protected by stable Si-C bonds and by a steric hinderance mechanism. The modified photonic structures are stable for many months.