The mechanical behaviour of sulfonated polystyrene ionomers can be altered by changing the nature of the counterion and by addition of excess neutralizing agent. Enhanced tensile strength and greater energy to fracture have been obtained by changing the counterions from monovalent Na to divalent Ca. The Ca ions create stronger ionic crosslinks that result in a more effective entanglement network. Increases in bath tensile strength and toughness have also been obtained in an Na salt sulfonated polystyrene ionomer by addition of a 100% excess of neutralizing agent. The enhancement in properties is thought to arise from the presence of a small, coherent second phase that acts as a reinforcing filler and increases resistance to crack propagation.