An experimental study has been performed to analyze the role of fluid viscosity in ultrasonic atomization when excited with waves in the megahertz range. In the present experiments, liquid mixtures of water and glycerin have been atomized with a piezoelectric disk, and the atomization rate has been evaluated as a function of kinematic viscosity. Although atomization has been observed for viscosities as high as 28 x 10(-6)m(2)/s, corresponding to a glycerin volume percentage of 80% at 35 degrees C, the atomization rate drops almost by a factor of 10 before reaching 5 x 10(-6)m(2)/s. Measurements have been obtained for a range of excitation voltages. Droplet size distributions have also been determined and they have been found to be nearly independent of both voltage and viscosity.