Freshly prepared porous silicon (PSi) surfaces have been modified with different organic molecules such as alkenes (1-decene, ethyl undecylenate) and nonconjugated dienes (1,7-octadiene) in a thermal process to yield organic monolayers covalently attached to the surface through Si-C bonds. These monolayers have been characterized using diffuse reflectance infrared Fourier transform, X-ray photoelectron, and Raman spectroscopies. Derivatized surfaces have shown a high stability in boiling aqueous and organic solvents, and in 49% HF and KOH (pH 13) solutions at room temperature. They protect the surface against oxidation when exposed to air at 100% humidity. The high passivation of the surface implies also photoluminescence (PL) stabilization. In fact, the modified surfaces retained their original PL, and the chemical process affected neither the peak position nor the intensity. The PL was preserved even after a steam treatment for several weeks, while the hydrogen-terminated porous layer was completely transformed into a transparent oxide.