The Sauerbrey equation, widely used in quartz crystal microbalances (QCM), expresses the relationship between resonance frequency changes and mass load changes. When a QCM is used in a liquid, however, its resonance frequency is known to be affected not just by the mass load but also by the viscous load of the liquid. Using a network analyzer, we analyzed the impedance of an AT-cut quartz resonator. Based on an admittance diagram given by impedance measurements, the series resonance frequency fs, where conductance is at its maximum, and the two frequencies f1 and f2 (f1 < f2), where conductance is half its maximum, were determined and analyzed at the same time. f1 and f2 are frequencies traditionally used when determining the quality factor for quartz. Focusing on these frequencies, we first conducted an experiment using a 27 MHz quartz resonator in which we changed the liquid temperature of de-ionized water from 26 °C to 62 °C. Changes in liquid temperature allowed measurement of the effect of viscosity changes in the de-ionized water on the above frequencies. Results revealed a rise in frequency of about 2500 Hz for fs and about 100 Hz for f2. Next, we measured the effects of each frequency when the liquid was changed to a glycerol concentration of 0–14 wt% and an ethanol concentration of 0–10 wt%. fs was affected by viscosity, which changed due to the rise in concentration. This frequency change coincided for the most part with theoretical values representing the viscous load of the liquid. f2, however, was mostly unaffected by viscosity, which changed as a result of the rise in concentration. Then, we performed measurement with a mixture of avidin and glycerol. The frequency change for fs was about −50 Hz while that for f2 was about −30 Hz. When, as a reference, a solution of avidin alone was added, the frequency change for both fs and f2 was about −30 Hz. When glycerol alone was added, the frequency change for fs was about −25 Hz, but there was no frequency change for f2. Based on theory and the above measurement results, f2 was found to have characteristics that are not affected by the viscous load of a Newtonian liquid. Thus, using f2 for a QCM subject to viscous changes in a liquid allowed more accurate determination of the mass load than fs.