Flow of water in Whatman #17 chromatography paper and the interpretation of the capillary suction time method for determining filterability of suspensions is examined. Filter paper water flow is described by well-established theory of water flow in porous media. This theory is based on material balance for the water and Darcy's law. Assuming that the matric potential or suction (arising from the interaction of water with the solid surfaces and their geometry) and the hydraulic conductivity of both the filter paper and the suspension are well-defined functions of the water content, one-dimensional capillary suction time is analysed from considerations of material balance and continuity of water potential on the paper/ suspension interface. Experiments agree with theory and illustrate that flow in the paper is unsaturated, with the advance of a wetting front in paper a function of the interaction of the sorptivity-matric potential functions of both the paper and the suspension. Extension of the approach to radial flow in the filter paper is too complicated to permit simple interpretation of wetting front advance. The capillary suction time method must remain a useful empirical tool for practicing engineers. Its weaknesses should be recognised, however, and constant pressure filtration tests, for example, which provide both sorptivity-, and equilibrium water content- pressure relations simply and accurately must ultimately be preferable.