We demonstrate efficient coherent anti-Stokes Raman scattering from Raman-active modes of toluene molecules in solution and gas-phase nitrogen molecules infiltrated into mesoporous silica aerogels. Due to their high porosity, high transparency in the visible, and weak scattering, silica aerogels are shown to provide an ideal host for Raman-active gases and liquids, detected and analyzed by coherent anti-Stokes Raman scattering, allowing the creation of gas- and condensed-phase sensors of chemical and biological species, including sensors of pollutants and aerosols. (C) 2004 Published by Elsevier B.V .