Surface-plasmon-enhanced diffraction of index-modulated polymer gratings are used to detect the smallest contrasts in the refractive indices of ultrathin polymer films. The dependence of the enhancement of the diffraction orders on the grating constant, metal and film thickness is studied in detail. The method is the two-dimensional analogue of the forced-Rayleigh-scattering technique, used for example to detect diffusion of bulk polymers undergoing a glass transition.