The formation of filter cakes is caused by filtration on the interface between soil and supporting fluid during the construction of diaphragm walls. The supporting fluid, basically a slurry of water and 4–8 % bentonite, is loaded with minerals from the soil while the trench is excavated. The filter cakes, mixtures of in situ soil and bentonite originating from the supporting fluid, are generally believed to be the weakest and therefore most significant transition zone concerning the shear force transfer from the soil to the concrete wall. Previous research has been devoted to the understanding of the composition and mechanical behavior of filter cakes. In this context, the content of bentonite in the filter cake was identified as the governing parameter. The current paper presents a novel concept to determine the amount of bentonite in the filter cake. Based on an iterative approach, the cation exchange capacity (CEC) of the filter cake, the pure bentonite, and the in situ soil is determined. A linear correlation between CEC and bentonite content in the range between 0 and 100 % could be established based on an extensive laboratory program for numerous soils and types of bentonite. This correlation can be applied to determine the bentonite content for a given filter cake. A comparison of the actual bentonite percentage of artificial sand-bentonite mixtures and the bentonite percentage determined by the CEC method revealed a good agreement (R2 ≥ 0.9825). In addition, filter cakes obtained on construction sites were examined, and a maximum deviation of ±0.84 % bentonite content was measured in duplicate tests.