Novel fine polymer particles containing ultrafine Pd particles dispersed on the surface of core shell [core, poly(styrene-co-acrylic acid); shell, PrPO4]-type microspheres were prepared by the emulsifier-free emulsion polymerization of styrene with acrylic acid followed by the addition of PdCl2 and a mixture of Pr(NO3)(3) and NaH2PO2. Pyrolysis of the resulting polymer particles at 900 degrees C provides organic polymer-free hollow particles (diameter, 473-586 nm) composed of Pd metal and PrPO4. Hydrogenation of 1-hexene, trans-1,3-hexadiene, trans-1,4-hexadiene, and 1,5-hexadiene using the resulting Pd/PrPO4 particles (reactant/Pd = 500 mol/mol) as catalyst revealed very high activity, especially for hydrogenations of 1-hexene and 1,5-hexadiene as compared with conventional Pd particles/Al2O3 and Pd particles on powdered PrPO4.