When only ethanol is used as the source of reducing agent, and when 20 v.% ethanol is used, the NPs show a λspr value around 524 nm which corresponds to 13.8 ± 3.9 nm NPs. As the amount of ethanol decreases to 19 or 18 v.% the λspr values increase to 524 nm and 526 nm respectively, corresponding to 12.1 ± 4.5 nm and 16.5 ± 4.4 nm, respectively, see Figure . The trend that the NP size increases and/or the size distribution increases when the amount of ethanol decreases is confirmed when 10 v.% of ethanol is used, for which the λspr value increases to 558 nm corresponding to 18.6 ± 12.1 nm NPs. This corresponds to a case where there is not enough reducing agent and the NP synthesis is less controlled . The poorer control over the synthesis is even more pronounced for 2 and 1 v.% ethanol where there is not enough alcohol to perform the reduction of Au III , as indicated by relatively featureless UV-vis spectra with an overall low absorption, e.g. very low A400 values compared to the other samples, see Figures . As a result, no STEM characterization was performed on those samples obtained with 1 or 2 v.% ethanol. A more detailed screening of the influence of low amount of glycerol is proposed in Figure together with repeated experiments that confirm the trend observed, see Figures S5-S7 and S9. These results indicate that the NP size decreases as the amount of glycerol increases to ca. 5 v.%, while the viscosity of the reaction mixture increases as the glycerol content increases . In conclusion, small amounts of polyol in the range 1-5 v.% helps controlling the NPs size towards smaller size as the amount of glycerol increases. Importantly, this small amount of viscous polyol does not change much the overall viscosity of the samples still obtained in relatively low viscosity media containing mainly water and ethanol.