Accurate values for the equilibrium sorption and the diffusion rates of penetrants in polymers are required for many important polymer production and processing operations. This paper demonstrates the use of the quartz crystal microbalance (QCM) as a means to readily produce diffusivity and equilibrium data. The QCM exploits the well-known piezoelectric effect in quartz crystals to measure the mass sorption of a penetrant onto a polymer film. Nanogram resolution of the penetrant mass results in extreme accuracy. The mass sorbed as a function of time provides the necessary information to calculate the diffusion coefficient of the penetrant in the polymer, and the equilibrium mass provides the penetrant solubility. The flexibility and accuracy of the QCM are demonstrated by examining several sets of data. A comparison of penetrant-diffusion data obtained from two different techniques (the QCM and gravimetric sorption via a quartz spring balance) is presented to demonstrate the accuracy of the QCM. The results from an industrial coating/penetrant system are also given to demonstrate the capacity of the QCM to measure the equilibrium state.