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Anomalies in the intensity of p-polarized light from concave diffraction gratings (Wood's anomalies) have been used to obtain surface plasmon dispersion curves for dielectric-metal layers on the grating surface. These include a 350/~ MgF 2 layer and A1203 layers varying from a few angstroms to over 800/~ on an A1 substrate. The wavelength range of the incident and diffracted light is from the visible to the vacuum ultraviolet (7,500--500 A). Anomalous polarization peaks for A1203 layers in the vacuum ultraviolet (at ~ 1,600/~) are shown to shift significantly to longer wavelengths for only a few angstroms (up to 50 A) of oxide thickness, while for thicknesses greater than ,-~ 50/~ any further shift is small. In the visible region (> 4,000/~), on the other hand, the wavelength shifts are small for dielectric thicknesses up to ,-~ 50 A, but are large for thicknesses of several hundred angstroms. These results are in substantial agreement with the theoretical dispersion curves for these cases. Also considered are some of the effects of diffusion pump oil.