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The theoretical basis for a testing tool in the form of a circular waveguide for measuring both the dynamic viscosity and the longitudinal bulk velocity of viscous liquids is presented. It is based on the measurements of the attenuation of the fundamental torsional mode, T(0,1), and the fundamental longitudinal mode, L(0,1), of the waveguide when immersed in the liquid. The modeling techniques to extract the shear viscosity and the longitudinal bulk velocity are explained and experimentally verified. Results for two viscous liquids are presented: good agreement between theory and experiment was obtained.