Effect of surface group on the electrocatalytic behaviour of 10 wt.% Pt-Fe-Co alloy-dispersed carbon (Pt-Fe-Co/C) electrode has been investigated as functions of applied potential and duration in 85% H3PO4 solution of 145 degrees C, using Fourier transform infrared (FTIR) spectroscopy, combined with nc-impedance spectroscopy, potentiostatic current transient technique, and potentiodynamic polarization experiment. It was shown from FTIR spectra that surface group formed in this work: mainly comprises carboxyl group and that the formation potential of carboxyl group lies between 600 and 700 mV(RHE). From increase of charge transfer resistance (R-ct), and decrease of electrocatalytic activity for oxygen reduction with immersion time, it is suggested that above the formation potential of carboxyl group, further formation of carboxyl group on the carbon support around the catalyst particle reduces active surface area of the catalyst particle with immersion time. On the other hand, below the formation potential, dissolution of carboxyl group previously formed on the carbon support raises active surface area of the catalyst particle. Ln the present study, relationship between electrocatalytic aspect of the electrode, and the amount of carboxyl group formed on the carbon support around the catalyst particle was well discussed with a schematic illustration. The illustrative representation is underlain by formation on and dissolution from the catalyst particle of carboxyl group which cause the rise and fall in circumferential coverage of carboxyl group, respectively and hence thr reduction and elevation in active surface area of the catalyst particle.