Gold nanoparticles with shapes which varied from spheres to multipods and polygons were prepared with a seedless approach in water/methanol mixtures in the presence of polyvinyl alcohol using sodium ascorbate as the reducing agent. The shape of Au nanoparticles is critically affected by the water/methanol ratio, as well as by the ratio of hydrogen tetrachloroaurate (HAuCl 4) to sodium ascorbate and the concentration of HAuCl 4 in the reaction mixture. A decreased ratio of water to methanol below 30/70 leads to the formation of multi-branched nanoparticles with the size in the range of 30-70 nm at relatively low HAuCl 4 concentration, whereas polygons are formed when HAuCl 4 concentration increases. The polyvinyl alcohol stabilized multi-branched Au nanoparticle colloids were stable at room temperature for a period of at least six-month. Copyright © 2006 American Scientific Publishers All rights reserved.