For polymer electrolyte membrane fuel cells (PEMFCs), the importance of durability is widely recognized, but less attention has been paid to the role of ionomers. In this study, the importance of ionomer structure in achieving high PEMFC performance and durability are investigated experimentally for different catalyst-ionomer ratios and catalyst loadings in scaled up cell (45 cm(2)). The results are compared with a conventional long side chain ionomer (LSC) under the same preparation and testing conditions. Catalyst layers (CLs) fabricated with 25 wt% of short side chain (SSC) ionomer display higher performance than 17 wt% and 30 wt%. A similar trend is also demonstrated when using the LSC ionomer. However, it is found that SSC ionomer is more compatible with CLs than LSC. This compatibility is ascribed to the higher stability of the SSC ionomer. In addition, higher performance, Pt utilization, and active surface area are measured for membrane electrode assemblies (MEAs) prepared with SSC in comparison to LSC under the same ratio. Based on the accelerated stress tests, SSC ionomer has a positive role in improving durability, as the maximum power density after 30,000 cycles decreases by 21% and 48% for MEAs prepared by SSC and LSC, respectively. Moreover, the losses in performance are more than two times greater than when the Pt loading is decreased from 0.5 mg/cm(2) to 0.125 mg/cm(2). These results highlight the importance of ionomer structure in cell performance and durability at high and low Pt loadings.