Many material processes may be usefully characterized by measurement of the effective viscosity during the solidification of the material. For example, monitoring the viscosity of a resin during the curing of a composite material allows the optimization of the cure strategy to minimize porosity in the final product. Also, the flow of fluids is essentially a function of their viscosity and well-regulated flow is often required, such as in an automated painter or in a printing press. Thus, it is important to have a method to measure viscosity continuously during the entire process time. It is well-known that viscosity can be determined by using ultrasonic techniques.1 Usually, this is done by measuring the attenuation of longitudinal waves propagating in the viscous medium, but this method requires many assumptions and many conditions to be fulfilled that obviously limit its applicability. Some of these assumptions and conditions are: (1) all the other causes of ultrasonic attenuation such as diffraction, dispersion, thermoelastic loss, must be negligible in comparison to the viscous loss; (2) the sum of the volume and the shear viscosities is measured, rather than only the shear viscosity;2 (3) usually, the ratio of the imaginary part of the stiffness coefficient to the real part of this same coefficient must be assumed to be much lower than unity; (4) the viscous medium must have sufficient thickness that the different echoes in the pulse-echo train are resolved, yet be thin enough that the first echoes are detectable; and (5) internal reflections inside composite laminates must be assumed negligible.