We investigate the properties of long-chain alkyl xanthate-capped platinum nanoparticles. First an uncapped platinum colloid, 4.0 +/- 1 nm in diameter, is produced in an aqueous phase, and then the capping agent is added. The capped particles are hydrophobic and easily transfer into organic solvents. They can be dried and repetitively transferred to various organic liquids in a reversible manner. Xanthate-capped platinum colloids are more stable than the analogous thiol-capped particles land certainly noncapped particles) toward chemical corrosion (by oxygen in the presence of cyanide ion) and are also stable on heating. These colloids exhibit a characteristic sharp absorption in the range 450-470 nm in addition to the extinction in the UV. This absorption might be assigned tentatively to a weak d-d transition of platinum observed in this region also for the PtCl62- salt and for its adduct with xanthate though it is absent for uncapped Pt particles. The transition for the capped colloid, however, is much stronger compared to that observed for the salts.