In this paper we show the possibility of measuring Raman spectra of thin layers using waveguide stimulated Raman gain spectroscopy (WSRG). In WSRG, the thin layer is probed by the evanescent field of the pump and Stokes beams as they propagate as guiding modes in an optical dielectric waveguide structure. We have measured the Raman spectrum of a 30 nm thin polystyrene layer that was deposited on top of a Si3N4 waveguide. The measured gain of the strong 1000 cm(-1) benzene ring stretch vibration was found to be 10(-3), which is about 5 orders of magnitude higher than for a classical transmission geometry. Additionally, we show the advantage of this technique over spontaneous Raman scattering in waveguides in the case of strongly fluorescing waveguides.