Pt particles have been prepared by reduction of hexachloroplatinic acid with ethylene glycol in the presence of a negatively charged polymer poly(N-sulfonatopropyl p-benzamide), The average size of platinum particles was investigated by transmission electron microscopy and found to be in the nanometer size range. The effects of the polymer content on the average particle size were studied. The average particle size of the Pt colloid prepared from solutions containing a polymer/Pt weight ratio of 20 and 0.2, respectively, varied from 2.45 to 5.9 nm,