The reduction of AuCl4- by citrate produces gold nanoparticles with a variety of shapes and sizes depending on the exact conditions used in the preparation. We present results that show that the pH of the reacting mixture has a dramatic effect on the size, polydispersity and morphology of the resulting gold nanoparticles. This occurs by altering the citrate charge controlling its stabilizing effect during the growth of the particles. On one hand, wide size distributions of predominantly polyhedra are obtained at pH lower than 5.0. On the other hand, narrow size distributions of nearly spherical particles are obtained at pH higher than 6. A mixture of ellipsoidal and other shapes with aspect ratio larger than one is formed when the reaction is performed at a pH in the 5-6 range. The mean particle diameters as well as the width of the distributions decrease monotonically as the pH of the solution increases settling to the well-known values for these colloids with a width of ca. 10% of the mean diameter. The variation in the size distribution is controlled mainly by the average charge per citrate molecule. (C) 2007 Elsevier B.V. All rights reserved.