This work presents a novel characterization method for distinguishing ionomer phase within porous composite catalyst layer. Quantifying ionomer distribution in a catalyst layer of polymer electrolyte fuel cells (PEFCs) is critical to understanding the morphological and transport properties of this key component. Current methods to image ionomer rely on staining it with heavy ions, such as cesium, Cs, to enable detection with hard X-rays, however if the catalyst layer contains electrocatalyst, such as platinum, Pt, then the challenge is to distinguish between Cs-stained ionomer and Pt. These two materials have very similar X-ray attenuation coefficients. Using synchrotron transmission x-ray microscopy (TXM) and two energy imaging, above and below Pt electrocatalyst absorption edge, the ionomer distribution is quantified. The method is applied to a PEFC catalyst layer and compared against the method of single energy imaging, which is not able to separate the two phases.