The structure and density of the d-state of Pt fine particles on a carbon electrode were investigated in H2SO4 and NaOH solutions by in situ X-ray absorption fine structure spectroscopy. The applied potential was changed between - 0.2 V and 1.3 V vs. Ag/AgCl for 0.26 M H2SO4 solution. The total density of the unoccupied d-state h(T) increased according to the potential and was always larger in the negative sweep than in the positive sweep. Around 0.0 V vs. Ag/AgCl in H2SO4, h(T) agreed in both sweep directions, that is, a kind of hysteresis was observed in this voltammogram. The coordination number and the disorder factor were determined for the first Pt-Pt shell. The disorder of the structure of the particles (decreasing N and increasing sigma) increased in the course of the positive sweep and disappeared at the hydrogen region after reversal of the sweep. The relation of the structural change to the hysteresis of the d-density is discussed with respect to cyclic voltammograms of Pt.