Femtosecond transient reflectivity measurements were performed for Au( I 11) electrode/aqueuos solution interfaces under electrochemical potential control. Electrode interfaces modified with and without a self-assembled monolayer of alkanethiols were studied in aqueous solutions of HClO4 and H2SO4. Dependence on electrochemical potential, interface modification, and kind of solute anions are observed concerning the transient reflectivity response in 3ps. For Au electrodes in an aqueous HClO4 solution, the rate of the initial relaxation at an unmodified interface, 6-10 ps(-1), decreased with the electrochemical potential at the potential range where a water dipole rearranged. The rate for alkanethiol-modified interfaces showed a tendency depending on the hydrocarbon chain length. A relative deposited energy from absorbed photon energy was estimated, indicating that interface modification with alkanethiols restricted energy dissipation from the interface. Our trial could contribute not only to develop a study of interaction between a substrate and adsorbed molecules in an ultrafast time scale but also to elucidate elementally processes of electrochemical reactions such as oxidation/reduction reactions at an electrode interface.