In this study the use of zeolites as ion-exchangers for equalization of ammonia peak loadings in aerated biological activated filters was investigated. For this use zeolites were mixed with a filter medium and the nitrifying filter was loaded with ammonia peaks for two hours at different flow rates. It could be demonstrated that during the phase of higher inflow concentrations the zeolite was collecting ammonia. When the influent concentration decreased ammonia was desorbed from the zeolite and could be nitrified by the bacteria growing on the filter medium. It was shown that additional zeolites can equalize variations of ammonia, especially in filter units which are working at a high nitrification rate and which are very sensitive to varying influent conditions. During the operation time of the filter no separation of the zeolite by higher hydraulic loading or by backwashing could be recognized. So the upgrading of nitrifying filters with zeolite represents an additional security for effluents containing oscillating ammonia concentrations.