Herein, we present our results related to the synthesis of colloidal solutions of gold nanoparticles (AuNPs) dispersed in castor oil. These colloids were prepared via a wet chemistry process by mixing specific amounts of castor oil, ethanol, and aqueous solutions of tetrachloroauric(III) acid and sodium hydroxide. The size and shape of the AuNPs obtained could be modulated by the amount of gold source added and the Au/OH^– molar ratio used. In this study, we observed that the free fatty acid content in the reaction medium was an important parameter to be considered in the syntheses of the colloidal solutions and the respective form and shape of the AuNPs produced. Thus, we evaluated the effect of oil acidity by adding different amounts of myristic acid (MA) in the reaction medium. The colloids were characterized by UV–vis spectroscopy, and the size and shape of the AuNPs produced were characterized by transmission electron microscopy (TEM).