Finely divided Pd particles (2-3 = in diameter) were synthesized by the reduction of Pd2+ precursor ions in the presence of cationic (myristyltrimethylammonium bromide, MTA(+)Br(-)) or anionic (sodium dodecyl sulfate, Na+DS-) surfactants. The protective adsorption layer of the ionic surfactants around the particles ensured the long-term stability of the aqueous dispersions. When the palladium hydrosol stabilized with MTA(+)Br(-) was mixed with an aqueous suspension of sodium montmorillonite, Na+MM- (a cationic clay), the cation-exchange reaction between Na+ and MTA(+) rendered the montmorillonite surface hydrophobic, in parallel with the incorporation of the released Pd particles into the MTA(+)MM(-) organoclay host. In a similar way, after addition of the palladium hydrosol stabilized with Na+DS- to an aqueous suspension of hydrotalcite nitrate, HT+NO3- (an anionic clay), the ion exchange between NO3- and DS- resulted in the formation of a hydrophobic clay, HT+DS-, with simultaneous deposition of the released Pd particles onto the clay lamellae. The low-loaded, highly dispersed Pd-organoclay materials displayed extremely high catalytic activities under mild conditions in the liquid-phase hydrogenations of styrene, hex-1-ene, and cyclohexene. Furthermore, the catalysts exhibited high selectivities for the partial hydrogenation of 1-phenyl-1-pentyne to 1-phenyl-cis-1-pentene. These high activities and selectivities were explained in terms of the high degree of dispersion of the I'd particles and the hydrophobic nature of the catalysts.