Tensile fracture behavior of sulfonated polystyrene (SPS) ionomers was investigated as a function of ion content. Both tensile strength and toughness increased slightly at low ion content (below ca. 4 mol %) and increased significantly at higher ion content above 4 mol %, although some reduction of these values was observed at an ion content of 8.8 mol %. The deformation mechanism of the thin film SPS ionomers also changed : from crazing only at low ion content to crazing plus shear deformation at high ion content. Furthermore, the width of hackle bands observed on fracture surfaces decreased with increasing ion content. These observations are interpreted as the effects of ionic aggregates on deformation and fracture behavior of ionomers : at low ion content, small ionic aggregates (multiplets) work as weak cross-links, while at higher ion content, large ionic aggregates (clusters) offer more effective cross-links, and these cross-links enhance craze stability, somewhat like covalent cross-links, leading to better fracture resistance under simple tension.