A new method for the study of the interaction between unlike macromolecules was developed, based on the motional resistance (R-m) response of a piezoelectric quartz crystal sensor to viscosity change in a contacting dilute polymer solution. The experimental setup, constructed with a 9-MHz AT-cut quartz crystal, a flowthrough detection cell, and an impedance analyzer, was shown to provide highly reproducible data at 25degreesC and a fluid flow rate of 1.3-1.6 mL/min. A quantitative relationship between R-m and the relative viscosity of the dilute polymer solution was derived. A model for using R-m to study the interaction was proposed. The interaction between polyethylene glycol (PEG) and poly(vinyl alcohol) (PVAL) was studied by this method with a thermodynamic parameter a. Experimental results indicated that the values of a for the blend PEG/PVAL were 0.094 and 0.086, corresponding to molecular weights of PEG of 10,000 and 20,000, respectively, and in agreement with those of capillary viscometry. The positive value of a showed that an attractive interaction existed between PEG and PVAL. An important feature of the new method was that it could be used in continuous measurement.