The reduction of Zn(II) ions at the dropping mercury electrode in 1 mol dm(-3) NaClO4 + 0.001 mol dm(-3) HClO4 with the addition of anthranilic or thiosalicylic acid is examined using an impedance method in wide potential and frequency ranges. It is concluded that the studied acids affect mainly the rate constant of the first electron transfer, caralyzing the process. On the other hand, the transfer of the second electron is inhibited. Catalytic activity of thiosalicylic acid is higher than that of anthranilic acid, resulting mainly from double layer effects.