Resonant grating-waveguide structures were used for the excitation of long-range surface plasmons. Resonance spectral bandwidths of 1.9 nm were experimentally measured in both the reflected and transmitted intensities from these structures. Numerical calculations indicate that interference rather than the usual surface plasmon absorption mechanism plays the dominant role in the resonance response when the thickness of the guiding metal layer in the structures is reduced below 10 nm. ©1997 American Institute of Physics.