Platinum (Pt) nanoparticles have broad application in automobile pollution control, sensors, and fuel cells. Single-crystal platinum particles over the range of nano- to micron-meters were synthesized at the Pt/SiC interface in high pressure water vapor at 1200 A degrees C. These particles exhibited a cube-octahedral shape with predominant (111) facets. Formation of the Pt particles is likely due to water vapor-facilitated oxidation of the platinum silicide, resulting from the interaction between SiC and Pt. Well-aligned Pt single-crystal particles with sizes of tens to hundreds nanometers were obtained on the surface of arc-melted Pt2Si after exposure in flowing water vapor (90 cm/min) at 1200 A degrees C for 5 min. The potential applications of this finding are discussed.