An experimental study is presented of the changes in residence time distribution in a granular bed during the course of deep-bed filtration, The results are described in terms of the distribution moments, and show that small amounts of deposit yield large changes in the second moment, with the first moment remaining practically unchanged, Higher moments were calculated, but could not be used because of the large experimental error. Evaluations on the basis of classical models of transport of solutes in porous media attribute these results to the hydrodynamic dispersion changes due to the accumulation of deposits. Interesting results were also obtained by varying the surface conditions of the filter. By turning the wettable glass beads into non-wettable beads the filtration efficiency was significantly enhanced.