Pt nanoparticles were obtained via the thermal decomposition of (NH4)(2)[PtCl4] (diammonium tetrachloroplatinate) by heating from room temperature to 760 A degrees C. The thermal decomposition process was analyzed using thermogravimetric analysis (TGA) and differential thermal analysis (DTA), X-ray thermodiffraction and infrared spectroscopy. The size and structure of the platinum particles were analyzed using transmission electron microscopy (TEM). The electrochemical activity of Pt particles was assessed by cyclic voltammetry in 0.5 M H2SO4. The TGA and DTA results suggested that the thermal decomposition of the precursor proceeded in two stages: loss of NH4Cl at similar to 300 A degrees C, followed by loss of NH4Cl and Cl-2 at similar to 372 A degrees C. Metallic Pt particles were then produced at temperatures of 372 A degrees C and above. At 760 A degrees C, the mean +/- A SD size of the Pt particles was (4.1 +/- A 1.6) nm, as determined from TEM measurements. In cyclic voltammetry (CV) measurements, an electrode comprised of glassy carbon and Pt particles in 0.5 M H2SO4 exhibited behavior similar to that observed using a polycrystalline Pt electrode.