Palladium colloid with a narrow particle size distribution is formed in the reduction of PdCl42- by hydrogen in aqueous solution, using sodium citrate as stabilizer. The absorption spectrum of the Pd particles depends on their content of chemisorbed hydrogen atoms. In the absence of hydrogen, the absorption maximum at 218 nm is present, which is expected from calculations. In the presence of hydrogen, the spectrum is flatter and peaks at 258 nm. Radiolytic methods are applied to prepare bimetallic PdcoreAushell and trimetallic PdcoreAushellAgshell particles, and the optical spectra of these structures are reported.