A perpendicular mesoporous platinum film is used as a model electrode to clarify the effectiveness of catalysts inside agglomerates of fuel cell catalyst layers on the basis of experimental facts. The analysis clarifies that: (i) Pt surface even apart from Nafion ionomer phase can be electrochemically active: (ii) its response is different from that of the surface covered with ionomer; and (iii) ionic resistance in pores filled with pure water is too high (ca. 0.18 M Omega cm) for fuel cell reactions to smoothly occur. We conclude that such catalysts in pores filled with pure water are ineffective for fuel cell reactions due to the high ionic resistance, though their catalytic activity is possibly higher than that of the catalysts covered with Nafion. (C) 2009 Published by Elsevier B.V.