The catalyst slurry used to form the catalyst layer of a polymer electrolyte fuel cell (PEFC) must be mixed for a sufficiently long period for good and stable cell performance. However, the optimum mixing duration must be determined from the viewpoint of process design. We prepared slurries with various amounts of Nafion and examined the influence of the slurry mixing time on the viscosity of the slurry, the size of the pores and distribution of elements in the catalyst layer, and the cell voltage of the PEFC. We found that when the Nafion content is optimum, these properties change gradually and stabilize after a sufficiently long mixing time as the progression of the adsorption of Nafion to catalyst particles. However, when the Nafion is comparatively low, although the properties stabilize after sufficient mixing, the Nafion molecules are first dispersed and then localized around aggregates. This reduces cell performance when mixing excessively long. These differences in the structural and electrochemical behaviors could be predicted by measuring the variation in viscosity during the mixing process. (c) 2009 Elsevier B.V. All rights reserved