The mechanism of rhenium-nickel electrodeposition on copper substrates from concentrated citrate baths was studied by examining the cathodic behavior of the bath components independently. It was shown that citric acid strongly adsorbed on the copper substrate inhibiting electron exchange involving foreign redox systems. Additionally, it was proposed that the rate determining step of nickel reduction was the dissociation of the nickel-citrate complexes. Such dissociation reactions were catalyzed by the hydrogen evolution reaction. A mechanism was proposed to explain the observation that the co-deposition of nickel increases the Faradic efficiency of rhenium electroplating. Published by Elsevier Ltd.