A new experimental technique has been developed in order to study the washing characteristics in a filter cake during displacement washing. The test material primarily used was paper pulp which forms highly heterogeneous filter cakes where the local values of, for example, porosity may be assumed to differ substantially from the average values. The experimental equipment used was a piston filter press equipped with a X-radiation source and a scintillation detector that was used to measure concentrations of different species inside the filter cake at different bed heights. In these experiments, Cs+ ions were used as the tracer compound for studying displacement washing. The filtrate was also collected and, from the displacement curves based on the concentration in the filtrate, the dispersion coefficients and the Peclet number could be determined; these values were used to calculate local displacement curves. The calculated local displacement curves agreed well with the measured local displacement curves showing that local measurement of the dispersion coefficient is a very valuable tool for studying the washing of filter cakes.