The metal particles of an interstitial solid solution of palladium carbide, PdCx (0 < x less than or equal to 0.15), were synthesized at room temperature in aqueous solution during the reduction of tetrachloropalladate(II) with sonochemically produced organic radicals. The number of carbon atoms in the Pd particles were controlled by changing the concentration and the type of organic additives. The following mechanism of PdC formation is proposed : (i) an active Pd cluster is formed during the synthesis of Pd particles, (ii) organic additives are then adsorbed on the Pd cluster surface, and finally (iii) carbon atoms on the particle surface, which are formed from the catalytic dissociation of the additives, diffuse in the Pd metal lattice.