The polarographic behavior of nitrobenzene and nitrophenols in the presence of a commercial quaternary ammonium salt {zephirol; $[C_nH_{2n+1}{cdot}N^{+}(CH_3)_2(CH_2C_6H_5)]Cl^{-}$,n = 8~18, Mol wt; 335} were examined. In the aqueous solution of $5{ imes}10^{-4}M$zephirol and 8 % ethanol, the half-wave potentials of these compounds were shifted 0~0.15 V more negative than those in the absence of the quaternary ammonium salt. The plot of the half-wave potential against the pH values of the solution is found to be liner, and the slope of these straight lines are equal to each other. The difference in half-wave potentials of