Ammonia borane (AB) has been used as a reducing agent in the deposition of cobalt on copper substrates. The bath composition for selective deposition was investigated. The mobile ion free plating bath was pH balanced and buffered using tetramethyl ammonium hydroxide. The codeposition of W with Co was achieved utilising tungstic acid. The electrochemical analysis of cobalt deposition and AB oxidation at the operating pH of 9 indicate that the deposition on copper is faster than that on cobalt and that the reaction rate decreases as the deposition proceeds.