The formation of uniform gold sols produced by the citrate reduction of auric acid is explored as a function of temperature and reagent concentration. As aurate ions are reduced, the reaction medium changes from black to purple to blue before turning deep red. These color changes are shown to result from a decrease in particle size over the course of the reaction. Electrophoretic and titration studies suggest the colloidal properties of the sols result from a combination of van der Waals, electrostatic, and short-range repulsive interaction potentials. Increases in particle surface potential over the course of the reaction are shown to result from competitive adsorption of citrate and aurate ions. A particle growth model incorporating colloidal stability is postulated.