The effects of polymer additives such as gelatin and polyethylene glycol (PEG) on Zn electrodeposition behavior were investigated by measuring the polarization curves for Zn deposition and the time-dependence of the cathode potential during long-term electrolysis. Gelatin and PEG act as polarizers for Zn deposition that shift the cathode potential to a less noble direction. The degree of polarization increased with increased concentration and molecular weight of the additives, while the polarization effect of the additives declined when their molecular weight exceeded 104. The polarization caused by the additives gradually disappeared with increased duration of electrolysis. The effective duration of additives increased with an increase in molecular weight and was longer in PEG-containing solutions than in gelatin-containing solutions. The longer effectiveness of PEG may be due to its stability in the electrolytic solution. The degree of polarization by the additives decreased with time, even when the additive-containing solutions were not subjected to electrolysis. The effective duration of additives was shorter in solutions subjected to continuous electrolysis than in solutions not subjected to electrolysis. On the other hand, the morphology and the crystal orientation of Zn obtained by long-term electrolysis were in good agreement with the change in cathode potential with time regardless of type of additive. With increased polarization of the cathode potential, the deposits showed a smooth surface composed of fine grains with preferred orientation of (1010). The deposits showed the same morphology as that in the additive-free solution when the polarization was decreased due to the degradation of the additives.