The frequency shift and the resistance change of a quartz crystal microbalance (QCM) coated with a viscoelastic film and working in a viscous liquid are analysed. It is shown that the measurement of the resistance provides additional information for an advanced signal interpretation. With the measurement of both values, viscoelastic contributions to the frequency response can be discovered. We present a method, which allows sufficiently accurate estimation of the complex sheer modulus of the coating. With this value the impedance approximation can be applied to calculate the film thickness more accurate than with those equations based on the assumption of thin rigid films.