The mass-frequency dependence for various materials deposited onto the surface of a resonating quartz crystal was investigated. The relationship is linear, but the constant of proportionality is different for different materials deposited. Such frequency changes are commonly described by the Sauerbrey equation, but in this experiment the equation was found to be in error by factors of up to 23. This can be accounted for by changes in surface viscosity. Slow temperature changes of a resonating quartz crystal coated with the polymer Antarox CO-880 revealed a significant frequency minimum at the phase-transition points. This is also attributed to a change in the viscosity of the coating at transition. The phase-transition temperature for this polymer was verified by differential scanning calorimetry. Quartz crystal techniques may provide a simple, accurate method for the measurements of phase-transition temperatures.