The present state of knowledge concerning the transudation of aqueous liquids into paper is reviewed. Theories for the swelling of cellulose and cellulose fibres are examined and related to the swelling of a porous web of paper. A method of measuring, simultaneously, the penetration and resultant swelling of the sheet by aqueous solutions is described. The results of the above measurements for water, water containing a wetting agent, starch solution, cuprammonium hydroxide solution and a glycerine/water mixture are reported. It is shown that the swelling is an integral part of the penetration process. Deviations from the Lucas-Washburn equation are due to the neglect in the penetration equation of the swelling of the sheet. Apparent diffusion coefficients are calculated from the swelling data and it is shown that swelling is controlled by a diffusion mechanism.