We have studied the mechanism of Pd autocatalytic displacement reaction on a silicon surface in both PdCl2-HCl-HF-NH4OH and PdCl2-HCl-HF baths. Using in situ and ex situ attenuated total reflectance-Fourier transform infrared (ATR- FTIR) and X- ray photoelectron spectroscopy (XPS) experiments, it has been shown that Pd(NH3)(2)Cl-2 in the electrolyte is reduced to metallic Pd particles by a displacement reaction and an (NH4)(2)SiF6 compound is generated during the Pd activation in PdCl2-HCl-HF-NH4OH electrolyte. From our results, we have concluded that different mechanisms of displacement reactions in each electrolyte result in the different characteristics of Pd growth. Hydrofluoric acid, which is generated in the Si etching reaction, is directly dissolved to SiF62- during the Pd displacement reaction. The SiF62- in the PdCl2-HCl-HF electrolyte plays a role as the Si etchant, whereas SiF62- bonds with NH4+ ion in the PdCl2-HCl-HF-NH4OH electrolyte. These different mechanisms result in the different of Pd growth speeds and characteristics. © 2005 The Electrochemical Society. All rights reserved.