Sb/Sb₂O₃ composites are considered promising materials for numerous applications, such as secondary batteries, catalysis and thermoelectrics. Recent research shows that nanostructured Sb/Sb₂O₃ micro-composites can be obtained straightforward by a single-step electrochemical deposition process. While macroscopically homogenous deposits are obtained in most cases it is shown herein that pattern with Sb and Sb₂O₃ rich regions emerge under specific conditions. The pattern formation is investigated using electrochemical methods combined with in-situ optical observation and electron microscopy. The origin of pattern formation is proposed to be related to the complex interactions between electrochemical Sb deposition and chemical Sb₂O₃ precipitation in combination with the emergence of convection patterns in the vicinity of the electrode. These findings contribute to a fundamental understanding in the nascent field of electrochemical metal/oxide co-deposition and can be useful for the rational design of advanced functional composite coatings.