AbstractWe report on the characterization of Cu2ZnSnS4 (CZTS) polycrystalline powder that was prepared in a facile one‐pot synthesis of colloidal nanoparticles and characterized with thermal analysis and X‐ray diffraction. For the investigation of impurities and trap states in CZTS, electron spin resonance (ESR) measurements were performed. Defect states for charge carriers as well as the existence of Cu(II) ions in varying amounts depending on the precursors used were observed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)