This paper presents a new model for predicting the evolution of the particle size of gold nanoparticles (GNPs) in the citrate synthesis method. In this method, the precursor is an acid solution of tetrachloroauric acid, while the reducing agent is a base solution of sodium citrate. The acid-base properties of the solutions influence how the size of the particles evolves during the synthesis. In the literature, various mechanistic theories have been proposed to explain this evolution. Turkevich et al. (1951), who pioneered this synthesis method, suggested the "organizer theory". This mechanistic description of the synthesis was modelled by Kumar et al. (2007), but recently Agunloye et al. (2017) showed that in several cases this model performed poorly, since it does not account for the acid-base properties of the reactants. In this work, we present a kinetic model based on the synthesis seed-mediated mechanistic description proposed by Wuithschick et al. (2015). In this description, the precursor concurrently reduces into gold atoms and hydroxylates into a passive form. The gold atoms then aggregate into seed particles, which finally react with the passive form of the precursor in a growth step. We validated the model using experimental data from the literature obtained for conditions in which the seed-mediated mechanism is valid. The predicted GNP final sizes closely agree with those obtained experimentally. (C) 2018 Elsevier Ltd. All rights reserved.