We examine if a microrheological interpretation of probe diffusion in model dispersions dominated by excluded-volume interactions and hydrodynamics captures the underlying viscoelastic relaxation mechanisms reasonably accurately. Standard dynamic light scattering is used to measure mean-squared displacements (MSDs) of visible probe particles in a refractive-index-matched model hard-sphere dispersion [poly(methyl methacrylate) particles in cycloheptyl alcohol]. The loss and storage moduli of the dispersion are extracted as functions of frequency omega from the measured MSDs. We suggest a semiempirical modification of the generalized Stokes-Einstein relation to convert the MSD to the viscoelastic modulus G*(omega). The results show a volume-fraction-dependent plateau G(infinity) at high frequencies in the storage modulus consistent with the domination of lubrication stresses. Viscoelasticity sets in at volume fractions phi above 0.2, and for 0.2 less than or similar to phi less than or similar to 0.45 the ratio of the mean viscoelastic relaxation time to the Peclet time tau(P) remains constant as had been observed previously for index-matched silica dispersions, but the microrheological measurements show a narrower spectrum of relaxation times. Master curves could be constructed for G*(omega)Ginfinity as sole functions of frequencies scaled with tau(P) in the above volume-fraction range. The microrheological method used provides moduli over a large range of frequencies from single measurements and avoids the need for time-temperature superposition to reach high frequencies. The effects of fast relaxation phenomena caused by soft surface layers and the deviations from hydrodynamic interactions expected for ideal hard-sphere systems can also be examined. (C) 2004 The Society of Rheology.