The application of polymer-based relative permeability modifiers (RPM) in low-permeability sandstone reservoirs has long intrigued production engineers. We hypothesize that lowering salinity may expand the application of the cationic polyacrylamide (CPAM) in low-permeability sandstone gas reservoirs, because of the favorable polymer-polymer physiochemical interaction at pore surfaces. To test this hypothesis, we conducted six core-flooding experiments to examine the relative permeabilities of water/gas before and after CPAM treatment, as a function of salinity using sandstone core plugs. Moreover, we measured zeta potentials of polymer-polymer, rock-polymer, and rock-brine interfaces before and after the RPM treatment, as a function of salinity. We also measured the contact angles for silicate/brine systems before and after the treatment as a function of salinity. Core-flooding results show that reducing the salinity decreases the water relative permeability but with marginal effect on the gas relative permeability. Lowering salinity also decreases the effective polymer layer thickness at pore surfaces. Moreover, the adsorption of the CPAM onto the negatively charged sandstone rock, together with lowering brine salinity, increases hydrophobicity of the rock. Thus, our results suggest that polymer-polymer interactions at pore surfaces govern the performance of CPAM in lowpermeability sandstone rocks.