Ammonia content is an important factor, which has to be considered when optimizing the processes in a biogas plant. A measurement system for a continuous monitoring of ammonia has been constructed, which can be placed in the gas room above the digester. The monitoring is based on the measurement of the electrical impedance. In this system, a condensate is generated on the surface of an interdigital electrode so that the whole stray field is covered by the condensate. For determining the ammonia content, impedance spectroscopy of these drops is performed. The ammonia of the biogas causes a decrease in the impedance in comparison to a pure aqueous condensate. A first evaluation of this measurement system using artificial slurries has been performed.