Poly (ethyl acrylate) of which molecular weight is about 100, 000 was hydrolyzed to controlled degrees with potassium hydroxide to produce compounds containing 70%, 80% and 90% carboxylate groups. Using these three water-soluble polymers as dispersing agents, sedimentation of pulp fibers in aqueous solution was investigated.At the more than about 0.016% polymer concentration, all the polymers were found to have dispersing capability decreasing the sedimentation rate of fibers. At the lower than about 0.008% concentration, however, the polymers have rather flocculating capability increasing the sedimentation rate than in pure water. The higher the extent of hydrolysis of the polymer is, the more the dispersing capability is in the concentrated solution and the less the flocculating effect in the dilute solution. On the contrary, the lower the extent of hydrolysis of the polymer is, the less the dispersing capability is in the concentrated solution and the more flocculating effect in the dilute solution.The polymers have no effect upon surface tension of the solution. But viscosity of the solution is high in proportion to concentration and degree of electrolytic character of the polymer. The viscosity is decreased after the sedimentation of fibers because of adsorption of the polymer on fibers.The dispersing capability of the partially caustic-hydrolyzed poly (ethyl acrylate) is discussed from a standpoint of molecular functionality and configuration.The number of ionizable groups introduced at intervals into the linear polymer chain brings about an appreciable chain extension by the electrostatic repulsion between the adjacent charged groups, and the solution viscosity increases. The extended polymer molecules are readily adsorbed on the fiber surfaces. The fibers are covered with a sufficient quantity of the adsorbed polymer in high concentration and the electrokinetic potential of the fiber surfaces is increased sufficiently to permit fiber dispersion, which is also promoted by the high viscosity of the solution. On the contrary, the polymer molecules hydrolyzed at lower degrees are somewhat coiled in the solution and the solution viscosity is not high, The fiber surfaces are insufficiently adsorbed with the polymers and their still low zeta-potential do not give good dispersing effect. In the dilute solution, contact and bridging of fibers are rather facilitated and the flocculation occurs, which in promoted by the low viscosity of the solution.