The capillary suction time (CST) method for measuring the filterability of a suspension is described, and its mode of operation is reviewed and discussed. A radial capillary suction time apparatus, and the filter paper used by it, are investigated in various ways to obtain the capillary suction pressure P, the water porosity epsilon of the filter paper, and the water saturation s(w)(r) in the filter paper as a function of the radial distance r from the suspension to be filtered. Air is shown to coexist with water in the wetted filter paper, but P and the effective water porosity are shown to be practically constant in the paper as used in the CST method. Measurements are made of the desorptivity S of a bentonite suspension using a conventional constant-pressure filter press, and compared with S obtained from CST measurements.