Asingle-molecule method has been developed based on the scanning tunneling microscope (STM) to selectively couple as eries of aniline derivatives and create azobenzenes. The Au-catalyzed oxidative coupling is driven by the local electrochemical potential at the nanostructured Au STM tip. The products are detected in situ by measuring the conductance and molecular junction elongation and compared with analogous measurements of the expected azobenzene derivatives prepared ex situ. This single-molecule approach is robust, and it can quickly and reproducibly create reactions for avariety of anilines.W ef urther demonstrate the selective synthesis of geometric isomers and the assembly of complex molecular architectures by sequential coupling of complementary anilines,d emonstrating unprecedented control over bond formation at the nanoscale.