In our previous work, we have proposed a new method to estimate the effective Pt utilization or "effectiveness" (Ef(pt)) using the ratio of the mass activity (MA) for the oxygen reduction reaction in the membrane-electrode assembly (MEA) in the polymer electrolyte fuel cell to that in the channel flow double electrode measurement, MA(max), under similar conditions. In the present research, applying this method, we have focused on elucidating the effect of the thickness of the catalyst layer (CL), the effect of Pt-based alloy catalysts, and effect of the state of dispersion of the Pt/C catalysts in the CL in measurements carried out at 80 degrees C and various relative humidities (RH), in either O-2 or air. The effect of a thin CL (0.04 mg cm(-2). Pt/C) has improved Ef(pt) by a factor of four, going from 3% to 12%, and the integrated effect of a thin CL and alloying (0.05 mg cm(-2) Pt3Co) has improved Ef(pt) by a factor of six, going from 3% to 17% for air at 0.85 V, T-cell = 80 degrees C, and 30% RH. Furthermore, we found that the Ef(pt) values were dependent upon the state of Pt dispersion in the CL The highest Ef(pt) value obtained thus far for air at 0.85 V, T-cell = 80 degrees C, and 100% RH was ca. 22%, shown by a low Pt loading CL diluted with added uncatalyzed carbon black (0.04 mg cm(-2), overall average 30 wt%-Pt). (C) 2011 Elsevier Ltd. All rights reserved.