The high frequency theology of model, hard sphere dispersions of charge-neutralized, coated silica particles in tetrahydrofurfuryl alcohol (THFA) was measured using two torsional resonators at five frequencies. The resulting elastic modulus shows omega(1/2) limiting behavior at high frequencies and is in quantitative agreement with the theoretical predictions of Lionberger and Russel [J. Rheol. 38 (1994) 1885]. The lack of a high frequency plateau is a signature of weaker hydrodynamic interactions acting at very small separations. Calculations verify that despite the lack of a high frequency plateau, these dispersions can exhibit reversible shear thickening at high shear rates, in agreement with experiment. Thus, the experiments verify the unique sensitivity of high frequency rheology to hydrodynamic properties at the particle surface.