A reverse symmetry waveguide is presented for evanescent wave sensing in aqueous solutions. The waveguide consists of a thin polystyrene film, supported by a thicker substrate layer of nanoporous silica on glass. The nanoporous substrate layer has a refractive index of n(S)=1.193, hence, with an aqueous cover refractive index of n(C)=1.331, a reverse symmetry (n(S)<n(C)) is obtained. A surface relief grating formed in the polystyrene film is used as a coupling element. Sensitivities considerably higher than for a conventional waveguide sensor are demonstrated. (C) 2002 American Institute of Physics.