The Electrochemical Quartz Crystal Microbalance has been largely used for the analysis of conducting polymers. Many groups quantify the mass increase or decrease on the surface of the quartz crystal directly from the changes in the resonant frequency of the crystal by applying the Sauerbrey equation. However this correlation can be used only if the material coated on the crystal is a thin, smooth and rigid layer. We treat the quartz crystal and the coating as a single resonator which can be represented as an equivalent electrical circuit (Butterworth-Van Dyke). The variation in the coating mass is compared to the changes in the inductance and the viscoelasticity is related to the resistance of the electrical circuit. Only in the case when the resistance is maintained close to zero throughout an experiment can the Sauerbrey equation be applied. The components of the equivalent electrical circuit are obtained by using the transfer function method, which is faster than an impedance measurement. The non-validity of the Sauerbrey equation is demonstrated for the hydration of a hydrophilic polymer film. The polymer swells considerably in water creating a very soft hydrated film. The use of mathematical simulations was then necessary in order to calculate of the rate of hydration, i.e. changes in the thickness of the film. We have applied the transfer function method on the Electrochemical Quartz Crystal Microbalance to study the electrodeposition and the redox switching of conducting polymers. The poly(aniline) films were electropolymerised in the presence of poly(vinylsulfonate) or poly(styrenesulfonate) counterions. The polymer films presented different morphology depending on the poly(anion) used during polymerisation. The polymer film which obeyed the Sauerbrey approximation was used in studies of ionic movement during voltammetric cycles in different inorganic acids. From experiments coupling the Electrochemical Quartz Crystal Microbalance with the Scanning Electrochemical Microscopy, protons were found to be responsible for maintaining charge neutrality on the poly(aniline)/poly(anion) composite film during redox switching. Water also moves into and out of the polymer film in the opposite direction to the motion of the protons.