Antimony (Sb) and antimony oxide have (Sb₂O₃) gained attention in research and industry in recent years because of their potential use in secondary batteries and as materials for sensor applications. As shown in recent studies, Sb/Sb₂O₃ composites with a variety of nanostructures and compositions can be manufactured by a straightforward electrochemical deposition process. In the present work the formation mechanism of Sb/Sb₂O₃ composites is investigated by means of electrochemical and chemical methods combined with electron microscopy and X-ray diffraction. A local pH shift, caused by the electrodeposition of Sb and leading to the co-deposition of Sb₂O₃, is verified experimentally for the first time. The Sb nucleation mechanism is investigated by fast transient measurements. The formation of Sb₂O₃ is examined by titration with KOH. Based on the results, a nucleation and growth model is proposed that explains the development of different Sb/Sb₂O₃ structures and compositions depending on the applied electrochemical process parameters.