Constant pressure filtration of kaolin slurries is studied by directly measuring the pressure drop across the filter medium under conditions leading to various extents of medium clogging. Experimental data are analyzed in terms of the conventional filtration theory based on the two-resistances-in-series model. The filter medium resistance and the cake resistance values obtained by the conventional data interpretation techniques, which require only the measurement of total filtration pressure, are compared with those calculated from directly measured pressure drops across the filter medium and the cake. The results show that the conventional analysis yields unrealistic filter medium resistances if the medium clogging is significant and continues throughout the course of filtration. The medium resistance is a significant part of the total filtration resistance even at long filtration times. The average specific cake resistance obtained by the conventional methodology does not appear to be in error when compared to its value calculated from the directly measured cake resistance at long filtration times. An actually incompressible cake, however, appears to be compressible due to cake clogging which increases with increasing filtration pressure.