Silver nanoparticles in aqueous solutions are most commonly synthesized by reducing silver ions with sodium borohydride in the presence of trisodium citrate, which acts as a capping agent. The net reaction, as well as the mechanism, is not yet definitively determined. Additionally, there is a degree of irreproducibility with respect to size distribution and stability. The current work provides insight into the mechanism of this reaction. The key difference between reactions that have been previously proposed is the production of H2. We have built an apparatus that allows us to monitor the progress of the reaction by sampling the gas phase species evolved. We show that hydrogen gas is in fact evolved during synthesis of silver nanoparticles. Through the use of isotopic substitution, we have found that both D2 and HD are produced during the synthesis of silver nanoparticles. We propose that D2 is the result of a Langmuir–Hinshelwood–type reaction and HD is the result of an Eley–Rideal–type reaction. Additionally, D2 is the major product of the catalytic decomposition of sodium borodeuteride on the surface of silver nanoparticles and HD is the major product of the formation of silver nanoparticles.