•Performance decreases with uneven cross-flow in higher channel length ARs.•Smaller performance change due to uneven cross flow than in experiment.•Neutron radiography indicates more water buildup in high AR than low AR.•Liquid water handling probable cause of majority of performance variation.•Significant interaction between channel AR and stoichiometry for system performance. The goal of this study was to determine the main cause of performance improvement in decreasing aspect ratio interdigitated flow fields, where aspect ratio is the channel length to width ratio. An experimental test cell with an interdigitated flow field operated under various aspect ratios found increasing net power densities with decreasing aspect ratio, after accounting for parasitic pump losses. In-situ neutron radiography found there was more water present in high aspect ratio flow field designs than in low aspect ratio designs. It also found more water in the 1.5/2 stoichiometry conditions than in the 2/4 stoichiometry conditions further indicating liquid water is a probable cause of performance changes. A single phase model used to determine the difference in power from the changing distribution of cross flow found decreasing aspect ratio resulted in higher overall performance. The experiment had significantly greater losses in power density with increasing aspect ratio, indicating liquid water removal was the major contributor to the improvement in net power density rather than the distribution of cross flow. Designers of PEMFCs should use a low aspect ratio design for interdigitated flow fields, or include a water removal mechanism for situations where high aspect ratio is required.