The QCM (Quartz Crystal Microbalance) is an ultra sensitive weighing device based on the piezoelectric, electromechanical oscillator principle. It consists of a thin single-crystal quartz disk, with one metal electrode deposited on each side. When the electrodes are connected to an electric oscillator, the crystal can be made to oscillate in a very stable manner at its resonance frequency, f. When a mass is adsorbed on one or both of the electrodes, then this leads to a change in the resonance frequency of the quartz crystal. If the adsorbed mass is small compared to the mass of the quartz crystal and there is no slip or deformation due to the oscillatory motion, then the resonance frequency decreases proportionally to the mass of the adsorbed film. It is possible to determine very small changes of the resonance frequency and hence very small mass changes, since the QCM generally has very stable oscillations. However, there are situations where the change in resonance frequency is not linearly related to the change in deposited mass, for example, when the mass is not rigid, slipping on the surface, or not deposited evenly over the electrode(s). There are also other things besides an applied mass that can influence the resonant frequency of a QCM including electrical effects, pressure, and temperature. In this diploma work the interaction between hydrophilic potassium mica surface and hydrogen mica surface and airs of different humidities have been investigated. The resulting adsorption isotherms show how much water that is adsorbed on the two surfaces.