The current-potential curve at a 2.8-nanometer-radius electrode immersed in a solution containing micromolar concentrations of a redox couple, [(trimethylammonio)methyl] ferrocene (Cp(2)FeTMA(+))/Cp(2)FeTMA(2+), shows discrete steps (a coulomb staircase) representing single electron-transfer events. Discrete potential steps are also found in the current-time curve as the electrode potential relaxes back to equilibrium after a coulostatic potential step.