The behavior of the surface free energy in the process of dyeing Leacril pretreated with tannic acid and subsequently dyeing with the cationic dye Rhodamine B has been studied. Also the electrokinetic behavior of these systems has been analyzed by studying the potential, which has been obtained by means of the streaming potential technique. Values more significative of the potential of these systems have been obtained using the three models of capillaries existing in the literature. The qualitative behavior of the potential is the same for the three models of capillaries tested in this paper. These models are those of Goring and Mason, Biefer and Mason, and Chang and Robertson. The potential of the systems analyzed is negative in the range of concentration of the dye in the liquid phase from 10 ؊6 to ca. 10 ؊4 M of dye. In the range of low concentrations (from 10 ؊6 to ca. 10 ؊5 M of dye) the potential of the system untreated Leacril/Rhodamine B increases in absolute value due to increasing hydrophobic attractions between both the hydrophobic chains of the dye and the Leacril fibers in aqueous media. In the system Leacril treated with tannic acid/Rhodamine B, this increase is also due to the presence of hydrogen bonding between the phenolic hydroxyl groups of the tannic acid and the sulfonate and sulfate end groups of Leacril fibers. For concentrations of dye between 10 ؊5 and 10 ؊4 M of dye in solution, the potential decreases in absolute value due to the electrostatic attractions between the groups negatively charged in the fiber and the cation of the dye. The potential changes its sign at the highest concentrations of dye used in this work. The adsorption of Rhodamine B onto both untreated Leacril and Leacril treated with tannic acid is favored by the increasing temperature of adsorption. The behavior of the components of the surface free energy obtained by the thin-layer wicking technique led us to consider that the cationic dye Rhodamine B is adsorbed on the surface of both untreated Leacril and Leacril treated with tannic acid by Lewis acid-base interactions.