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In the present work, we employ “nanometal-on-semiconductor” SERS substrates to detect various explosives including TNT, PETN, RDX, and TNB in vapor phase at concentrations on the order of few ppb. The SERS substrates were prepared by a novel process, where a monolayer of surfactant-free Ag nanoparticles was reduced on a Si thin film. Our work also targets a clear understanding of the factors, which so far have limited vapor phase SERS of the explosives. In addition to low vapor pressure of explosives at room temperature, we identify other challenges on the basis of our kinetics study: 1) weak adsorption of the explosives on silver nanostructures that accounts for easy desorption even under low laser excitation; 2) photochemical decomposition of the explosives under the Raman laser; 3) multiple chemisorption configurations of an explosive molecule on the nanoparticle surface that complicate the SERS spectrum. Although SERS signals of the explosives show transients, reproducible fingerprint spectra can be obtained in the first 10 s.