Using oxalate as a stabilizing agent, stable colloidal solutions of platinum nanoparticles of different shape distributions were prepared by reducing K-2[Pt(C2O4)(2)], K2PtCl6, or K2PtCl4 with hydrogen. The mean diameters of the Pt nanoparticles prepared from these precursors are 6.5,3.5, and 7.9 nm, respectively. UV-vis absorption spectroscopy and transmission electron microscopy studies on the preparation processes and the products indicate that not only the rate of reduction and particle size but also the shape distributions of the prepared Pt nanoparticles depend on the used precursors. The reduction of K-2[Pt(C2O4)(2)] in water at room temperature is quite slow, while adding suitable amounts of CaCl2 or increasing temperature can accelerate the reduction process. Pt nanoparticles prepared from K-2[Pt(C2O4)(2)] have quite narrow size and shape distributions, and the selectivity of cubic nanoparticles is greater than 90%. Adding CaCl2 in the reaction system does not obviously affect the size and shape distributions of the prepared nanoparticles. Oxalate-stabilized Pt colloid is sensitive toward oxygen; during aging in air, oxidation decomposition of oxalate is catalyzed by the Pt nanoparticles, which leads the Pt nanoparticles to aggregate into linear aggregations. After exposure to air for long times and treatment with hydrogen, these linear aggregations transform into Pt nanowires.