The time-dependent inhibition of the copper deposition reaction on Ru surfaces was investigated electrochemically at rotating-disc Cu and Ru electrodes in the presence and absence of organic plating additives. The copper deposition rate is high on bare ruthenium. It decreases as the ruthenium surface becomes coated with copper and as the plating additives adsorb and eventually reaches the slow steady-state copper deposition kinetics on copper itself. This differential inhibition effect (DIE) of copper additives during direct electroplating on a foreign substrate (i.e. not copper) is investigated with voltammetry and potentiometry at a rotating Ru disk electrode, and compared with a rotating Cu electrode. We also use simultaneous electrochemical measurements and in situ transmission electron microscopy (TEM) to study nucleation and growth of copper islands electrodeposited on ruthenium electrodes. Combining the in situ TEM and electrochemical measurements allows us to observe the growth of individual islands in real time and establish the effects of additives on island shape and saturation island density.