The mode of action of aseorbic acid in ferrous ion-plating solutions were investigated with the polarographic, potentiodynamie and electrolysis methods. Experimental results showed that ferric ion was reduced to ferrous ion by ascorbic acid and the concentration of dissolvd oxygen was depressed to very low value. The products of ascorbic acid oxidation could be reduced back to the original compound at cathode. Therefore, ascorbic acid concentration in plating solutions could be maintained at constant level, thus the oxidation of ferrous ions is effectivelly suppressed.