A new method for the determination of polymer viscosity-average molecular weights was developed with flow piezoelectric quartz crystal (PQC) viscosity sensing. The experimental setup with a 9 MHz AT-cut quartz crystal and a flow detection cell was constructed and shown to be able to give highly reproducible data under the temperature of 25 +/- 0.1 degreesC and the fluid flow rate of 1.3-1.6 mL/min. A response model for PQC in contact with dilute polymer solutions (concentration (<0.01 g/mL) was proposed in which the frequency change from the pure solvent, Deltaf(s), follows Deltaf(s) -k(6)eta (1/2)(1) + k(7), where eta (1) is the absolute viscosity of dilute polymer solution and k(6) and k(7) are the proportionality constants. This model was examined with poly(ethylene glycol) samples (PEG-20000 and PEG-10000) under the aforementioned experimental conditions using water as solvent. The result was Deltaf(s) = -1587 eta (1/2)(1) + 1443. Based on this model, the method for the determination of polymer viscosity-average molecular weights, M-eta, by flow PQC viscosity sensing was described and examined with an unknown poly(vinyl alcohol) (PVAL) sample. The new method proved to be an attractive and promising alternative for the determination of polymer molecular weights based on the good agreement between the molecular weight determined by the new method (M-eta = 58600) for the unknown PVAL sample with that determined by the conventional capillary viscosity method. The new method has some advantages over the conventional viscosity method; for examples, operation is simpler and more rapid; the instruments required are cheaper and portable; the needed sample quantity is smaller; and the experimental setup constructed can be used in continuous measurement.