The atomic arrangements inside Ru and Os nanoislands spontaneously deposited on Pt( 111) electrode surface were observed with electrochemical scanning tunneling microscopy. The surperlattice of the pristine Ru nanodeposits is (root 3 x root 3) R30 degrees-RuO+. Upon reduction, the Ru nanodeposits are compressed to a uniaxially incommensurate (root 3 x root 2)R30 degrees-Ru structure, which does not change during the following reoxidation. The atomic arrangement inside the pristine Os nanodeposits is an incommensurate (root 2 x root 2)-OsO+ structure, which does not transform during the subsequent reduction-oxidation cycles. The structures of the Ru and Os nanodeposits are discussed in terms of removal and insertion of oxygen ions depending on electrode potential.