The ionomer content in catalyst layers has a marked influence on direct methanol fuel cell (DMFC) performance. In an anode which contains unsupported PtRu as the catalyst, the recast ionomer may not always be necessary because the protonic conductivity of hydrous RuOx, the presence of which is inferred from the X-ray diffraction pattern, may be sufficient to allow effective utilization of catalyst sites. To examine interpenetration of catalyst and membrane material as a possible explanation for the lack of an apparent need of added ionomer, ultramicrotomed thin sections of the membrane-electrode assembly (MEA) were examined by scanning electron microscopy. Microscopic examination of such MEA cross sections revealed significant porosity in layers made by mixing unsupported catalysts with recast ionomer Images of such sections did not reveal significant interpenetration, supporting the interpretation that hydrous RuOx may by itself provide sufficient protonic conductivity in PtRu catalyst layers prepared with no added ionomer. In contrast we: show that the presence of recast ionomer in DMFC cathodes based on unsupported Pt as the catalyst is essential for optimum DMFC performance, because the recast ionomer is the primary source of protonic conductivity in the latter case. Having shown its potential function as proton conductor, we stress that Ru oxide is apparently not the key for maximizing DMFC anodic activity.