Hypothesis: Polymer additives such as Polyvinylpyrrolidone (PVP) are ubiquitously used in wet chemical reduction methods to tune nanoparticle sizes and shapes. However, all polymers retain some traces of their synthetic history through their end-groups and impurities. These impurities may thus impact redox and interfacial processes occurring during the formation of nanocolloids. Experiments: We report a systematic comparison of four representative silver nanoparticle syntheses in the presence of either commercial PVP or its purified version, obtained through dialysis or filtration. We characterized the resulting nanoparticle dispersions through UV-visible spectroscopy, electron microscopy, X-ray scattering and Raman spectroscopy. Findings: For all syntheses and methods, the simple removal of PVP molecular impurities drastically modifies nanoparticle size, shape and formation kinetic. Impurities from additives thus play a pivotal role in nanoparticle syntheses and must be systematically evaluated for relevant mechanistic investigations and robust process engineering. (C) 2020 Elsevier Inc. All rights reserved.