The structure of solid polymer electrode and catalyst ink of fuel cell has been investigated by focusing- (FSANS) and contrast-variation small-angle neutron scattering (CV-SANS). The solid polymer electrode, consisting of carbon (C), platinum, and ionomer (polymer, P), exhibited a power-law function with two asymptotes, i.e., from I(q) ∼ q−1 to I(q) ∼ q−4 with a crossover around q ≈ 0.005 Å−1. The scattering functions of the catalyst ink, i.e., the polymer electrodes dispersed in water, were successfully decomposed to the corresponding partial structure factors, SCC(q), SPP(q), SCP(q), exclusively representing C-C, P-P, and C-P correlations. SCC(q) was a monotonic decreasing function of q, dominating in the scattering from carbon clusters. On the other hand, SPP(q) exhibited a scattering maximum characteristic of polyelectrolyte solutions. This suggests that ionic clusters in polyelectrolyte solutions are formed in catalyst ink. The cross term, SCP(q), indicated that the carbon scattering is dominant and significant amount of ionomer is adsorbed on the carbon agglomerates. It is concluded that the catalyst ink consists of carbon agglomerates surrounded by ionomers and the presence of ionic-cluster path plays a key role in the performance of the solid polymer electrodes in polymer electrolyte fuel cells. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39842.