The resistance to fracture under alternating cycles of stress is examined for sulfonated polystyrene (SPS) ionomers. The influence of ion content on the number of cycles to failure and on fracture surface morphology has been determined. At low ion contents below 3 mol%, the fatigue resistance was found to be essentially unaffected, or slightly increased; but, in the range of ion contents from 4 to 6 mol%, cycles to failure increased appreciably. This behaviour, rather similar to the known influence of ion content on tensile strength, is attributed to the importance of a growing ionic cluster phase that leads to an increased strand density. Fatigue tests were also run on two series of blends of the SPS ionomer with polystyrene. In the first series, as the concentration of the SPS ionomer component (2.65 mol%) was increased, the average fatigue lifetimes were reduced with an apparent minimum at 30 wt%. In the second series of blends, containing IO wt% of SPS ionomer, the fatigue resistance decreased as the ion content of the ionomer was increased. These results are discussed in terms of the two-phase nature of the blends and the influence of alternating stressing on the extent of bonding between the SPS ionomer particles and the polystyrene matrix.