[en] Reduction of Tl⁺, UO₂⁺⁺ and Co⁺⁺ at the dropping mercury electrode in presence of 9-fluorenone (9-fl) in aqueous sodium perchlorate were studied. Tl⁺ give two polarographic waves, these waves increase with increasing concentrations of Tl⁺. A linear relatonship was obtained for the plot of I_d vs. C_Tl+ for the first wave, showing that this wave can be taken for the determination of Tl+ in this media. Uranyl ion is reduced giving a polarogram consisting of two waves. The first wave represents the reduction of UO₂⁺⁺ from +6 → +4 state, and the second wave represents the reduction of UO₂++ from +4 → 3 state. Co++ give a polarogram consisiting of one wave, E_1/2 of this wave = -1.40 V as. SCE. (authors)