Abstract <p>The effect of graphene oxide nanoparticles on the electrodeposition of a tin–bismuth alloy has been studied. Graphene oxide was prepared by oxidation of natural graphite in a mixture of concentrated sulfuric acid and potassium permanganate. X-ray phase analysis showed that the resulting nanoparticle suspension contained only graphene oxide, and no inclusions of unoxidized graphite were detected. It was established that the introduction of nanoparticles into an acidic electrolyte for tin–bismuth alloy electrodeposition resulted in the formation of time-stable solutions, in which no precipitate was observed during storage for four months. Reduction of the graphene oxide surface and its transformation into chemically reduced graphene were observed. The reduction most likely occurs due to oxidation of tin to Sn⁴⁺. The study showed that the addition of graphene oxide shifts the cathodic potentials toward more negative values over the entire current density range. The presence of nanoparticles exerts a greater effect on the electroreduction of bismuth than on that of tin, resulting in a decrease in the bismuth content of the composite coating. It should be noted that the addition of graphene oxide increases the cathodic current efficiency from 86 to 94%. Electron micrographs of the coating surface demonstrate a significantly smaller grain size of the composite coating, as well as the absence of large tin dendrites. X-ray structural analysis showed that the addition of graphene oxide somewhat promoted the growth of Sn grains along low-energy, low-index planes, whereas grain growth was suppressed along high-energy, high-index planes.</p>