Poly(vinyl pyrrolidone)-capped multiple twinned gold (Au) particles with decahedral shape have been synthesized by a simple and convenient solvothermal wet chemical method. In the process, hydrogen tetrachloroauric acid (HAuCl(4).3H(2)O) was reduced by ethylene glycol (EG) to form the multiple twinned Au nanocrystals in the presence of poly(vinyl pyrrolidone) (PVP) molecules at 200 degrees C under the extra condition of autogenous pressure. The decahedral nanoparticles take up about 10% of the total amount and have the usual size distribution from several tens to hundreds of nanometres. Some larger microsized five-twinned Au particles with perfect decahedral shape have also been observed in the final product. Furthermore, x-ray photoelectron spectroscopy (XPS) measurements verified that PVP molecules are adsorbed on the surface of the Au particles. Based on the experimental results, a growth mechanism has been suggested to elucidate the formation of the small decahedral Au nanoparticles as well as their evolution into perfect large decahedral Au particles with the size of several micrometres.