Measurements of dynamic shear impedance at 40 MHz and room temperature are reported for two polystyrene-solvent systems as a function of concentration. These measurements were made by an ultrasonic reflectance technique. The polymers had sharp molecular-weight distributions, M̄w/M̄n∼1.08, and molecular weights of 2.39×105 and 2.67×105. The concentrations ranged from about 1% to 20% and 3% to 30% in di-n-butyl phthalate, DBP (a near theta-solvent) and in toluene (a good solvent) for the two polymers, respectively. Results are reported in terms of the in-phase, G′, and out-of-phase, G″—ωηs, components of the dynamic shear modulus of the polymer and in terms of the reduced dynamic viscosity (η′—ηs)/(η—ηs), where η′ is the dynamic viscosity, ηs is the solvent viscosity, and η is the steady-flow solution viscosity. Results on the polystyrene—toluene system indicate that the system undergoes a change from Zimm-like to Rouse-like behavior with increasing concentration and, hence, with reduced frequency. No evidence of a reduced high-frequency limiting viscosity is observed and the value for the lowest concentration falls on a line extrapolated from earlier data of Lamb and Matheson. Results on the polystyrene DBP system indicate that the slope of logG′ vs log volume fraction of polymer increases at about 5% concentration, the predicted concentration for the onset of entanglement coupling. As concentration increases, a reduced high-frequency limiting viscosity is present up to the entanglement concentration, but beyond this point the reduced dynamic viscosity decreases and indicates a trend toward Rouse-like behavior at 20% concentration. Within experimental uncertainty neither solvent exhibits viscoelastic behavior.