The electrodeposition of AuCu/B4C composites from alkaline baths containing free cyanide is described. The electrochemical behaviour of the bath and the metallographic and crystalline structures of the electrodeposited alloys were studied with and without addition of particles. Electrochemical instabilities were observed and their bearing on the structure of both pure matrix and composite electrodeposits is shown and elucidated. The electrochemical and structural conclusions on the AuCu/B4C system are thought to be general for alloy-matrix composite plating with nonconductive particles.