Pd/SiO2 and Pd-Ag/SiO2 catalysts prepared by incipient wetness impregnation method were studied by using the techniques of microcalorimetric adsorption and infrared spectroscopy for the adsorption of H-2, CO and ethylene at room temperature. It is found that most sites of H-2 adsorption on Pd-Ag(1:1)/SiO2 and Pd-Ag(1:4)/SiO2 are weaker than those on Pd/SiO2 though the initial heat of H2 adsorption on Pd/SiO2 is not affected by Ag. The initial heat of 135 kJ mol(-1) for CO adsorption on Pd/SiO2 is the average heat produced by both bridged and linearly bonded CO on Pd while the initial heat of 117 kJ mol(-1) on Pd-Ag(1:1)/SiO2 and 90 kJ mol(-1) on Pd-Ag(1:4)/SiO2 are contributed by the linearly bonded CO on Pd sites modified by Ag with different Pd/Ag atomic ratios. These results suggest that the Pd-Ag/SiO2 catalysts may have well mixed Pd and Ag atoms in the Pd-Ag clusters. Strong interactions between the two types of atoms were observed. Ethylene adsorption on Pd/SiO2 at room temperature produces adsorbed ethylidyne and atomic hydrogen with the initial heat of 175 kJ mol(-1). Addition of Ag may inhibit the formation of ethylidyne and promote the formation of pi -species for ethylene adsorption in Pd-Ag/SiO2 catalysts. The formation of the pi-and di-sigma species for ethylene adsorption on the Pd-Ag(1:1)/SiO2 catalyst at room temperature generates the initial heat of 100 kJ mol(-1). The initial heat of 74 kJ mol(-1) produced for the adsorption of ethylene on the Pd-Ag(1:4)/SiO2 at room temperature may be attributed to the formation of only pi -species on Pd with more surrounding Ag atoms. This work demonstrates how the nature and bond strength of the surface species formed on the adsorption of ethylene on Pd may be altered by using Ag as a modifier.