This study describes the formation of functional organic monolayers on hard and soft interfaces via a vapor phase carbene insertion into Si-H and C-H bonds. We demonstrate that functional diazirine molecules can be used to form monomolecular coatings on silicon, silicon nitride, and urethane acrylate polymers under mild vacuum conditions and exposure to UV light. We investigate the molecular coverage and the long-term stability of the resulting monolayers in air, isopropanol, and water. Our results suggest that vapor-phase carbenylation can be used as a complementary technology to the traditional self-assembly, permitting functionalization of various passivated substrates with stable and functional molecular coatings under mild and scalable conditions.