The impregnated platinum catalysts showed various platinum particle sizes depending on the nature of the platinum precursors (Pt(NH3)(2)(NO2)(2) versus H2PtCl6) and on the pH of the Al2O3 suspension. The average platinum particle size increased with decrease in pH of the suspension in case of Pt(NH3)(2)(NO2)(2), but this trend was vice versa for H2PtCl6. The product distribution in hydrodechlorination (HDC) of CCl4 varied greatly with the platinum particle size; the larger the platinum particle size was, the higher was the selectivity to CHCl3. To elucidate the origin of this platinum particle size effect on product distribution, CO chemisorption, NH3 and CO2 temperature-programmed desorption (TPD), high resolution transmission electron microscopy (HRTEM), temperature-programmed surface reaction (TPSR), Fourier-transformed-infrared spectra (FT-IR) and X-ray absorption fine structure (XAFS) experiments were carried out. The formation of completely dechlorinated CH4 was favorable owing to the strong chemisorption of CCl4 on the small platinum particles characterized by low surface coordination numbers and by an electron-deficient property. The nature of carbonaceous species formed on platinum surface at the beginning of reaction also varied greatly with platinum particle sizes and changes of electronic state of platinum particles affected catalytic activity and products' distribution.