Very stable co-polymer (vinyl acetate (VA)-co-3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) (p(VA-co-DMAPS)) latexes with different compositions have been synthesized by emulsifier-free emulsion co-polymerization. The dry p(VA-co-DMAPS)s have been used in the preparation of drug tablets for sustained Metoprolol tartrate release. It has been shown that the tablet swelling depends on the mol fraction of DMAPS monomer units (m(DMAPS)), pH and ionic strength (I). An original explanation, based on the swelling behavior of p(VA-co-DMAPS), has been proposed for the "overshooting" phenomenon observed. It assumes the formation of hydrophilic domains with a higher m(DMAPS) in the co-polymer tablets. The formation of dipole-dipole clusters between the DMAPS units at different m(DMAPS) and I are the main cause for the established differences in both the swelling kinetics of the p(VA-co-DMAPS) matrices and Metoprolol tartrate release. The obtained results show that for p(VA-co-DMAPS) matrices-based tablets controlled sustained Metoprolol tartrate release can be realized just by varying two parameters, co-polymer composition and I.