The reproducibility of the synthesis of Gold NPs (AuNPs) as reference material candidates was investigated. This material class is particularly interesting because of its unique optical properties and the resultant use in bioimaging, biomedical applications and microelectronics. Moreover, AuNPs with suitable surface ligands are highly colloidally stable in water, provide excellent contrast in electron microscopy, and strong scattering of visible light and X-Rays, making them very suitable as reference materials for nanoparticle sizing methods such as scanning or transmission electron microscopy (SEM and TEM), dynamic light scattering (DLS), or small angle X-Ray scattering (SAXS). AuNPs stabilized with citrate were synthesized as described in the experimental section and the reproducibility of the synthesis was assessed by analyzing three different batches (AuNP1, AuNP2, AuNP3). From the DLS measurements, the average intensity weighted (di), volume weighted (dv) and number weighted size distributions (dn) revealed consistently similar hydrodynamic diameters, size distributions, distribution widths (polydispersity index; PI) and Z-averages (Figure ). The mean values and standard deviations were calculated as 23.0 ± 0.7 nm for di, 15.1 ± 0.7 nm for dv, 11.7 ± 0.9 nm for dn and 19.1 ± 0.9 nm for Z-average for the three batches, confirming a very small standard deviation and high reproducibility (see Figure ) and Figure ). Since DLS measurements can be somewhat sensitive to changes in concentration, colloidal stability, and pH, the slightly different intensities and peak positions can most likely be attributed to differences in concentration of the AuNP dispersions or small aggregates present in the nanoparticle dispersion.