This paper focuses on a method to determine the swelling pressure of polymeric hydrogels. The method is based on determining the swelling pressure at a defined volume of swelling that is smaller than the volume the gel can attain by free swelling to equlibrium. Data for one type of polymer particles made from weakly cross-linked and partly neutralized poly(acrylic acid) are presented for swelling in deionized water. For polymer volume fractions (phi(P)) in the range of 0.05-0.30, swelling pressures in the range of 0.25-4.31 MPa were obtained. The kinetic data are discussed with a combined diffusion-relaxation model for low and medium polymer volume fractions. A novel model is presented for higher polymer volume fractions. The mechanism of the gel-blocking effect that is evident in this phi(P) range is explained and considered in the novel model, in the form of a blocking factor. The results obtained may be used for the design of improved sealing concepts that are based on "super absorbing polymers".