Ionomer is a key material of proton exchange membrane fuel cells, not simply as it is responsible for proton conduction but as it affects both the local and bulk oxygen transport process in catalyst layers. In this work, a dual-layer cathode design is used to measure the local and bulk transport resistances in the catalyst layer with various ionomer contents in order to provide quantitative details about the effect of ionomer on oxygen transport process. The results reveal that both local and bulk resistances would enlarge with an increase in ionomer content which can be attributed to the variation of ionomer morphology. (C) 2019 The Electrochemical Society.