In the case of [ethanol+glycerol], the same phases are observed but the reaction proceeds faster than when only ethanol is used. Importantly, the time normalized data from 20 v.% ethanol and [ethanol+glycerol] with 18 v.% ethanol and 2 v.% glycerol follow the same general dynamic illustrated in Figure . This suggests that glycerol mainly plays a role to accelerate the initial steps of the synthesis. Since all conditions lead to approximatively the same final (Aspr) 1/3 values , and assuming that using different alcohols does not change too much the properties of the medium surrounding the Au NPs and hence their optical properties, and given the larger size of the resulting NPs when ethanol only is used, as detailed in Figure , it can be concluded that the number of final NPs obtained in presence of glycerol is higher than without. Therefore, the difference observed can be interpreted as follows: small amount of glycerol favors a faster formation of the initial seeds. Assuming that the classical nucleation theory applies here , this fast formation of the seeds followed by a slow growth leads to the observed smaller and more monodispersed NPs compared to the case where only ethanol is used (where the use of ethanol only is characterized by a slower nucleation process). Ethanol plays a role by providing extra reducing agents for the further growth and/or stabilization. A direct consequence of the fastest initial formation of the Au NPs is to develop more robust syntheses, e.g. less sensitive to the influence of light as detailed in section 5 in SI. The fast formation is observed with amount of glycerol as low as 2 v.%.