Carbon monoxide oxidation electrocatalysis and the subsequent surface reorganization on an electrochemically prepared PtZn alloy are presented. Zn was electrochemically deposited on a bulk Pt substrate and the alloy was naturally formed. This makes the surface of Zn-modified Pt (or PtZn) electrode rough compared to the surface of the original Pt substrate. CO oxidation catalysis on PtZn was measured and compared to that from Pt with a smooth surface. Although Zn modification did not enhance the catalytic activity compared to Pt, cycling the potential in a CO saturated acid solution appeared to make the PtZn surface smoother and Pt-rich. Oxygen reduction reaction (ORR) catalytic activity was also examined and compared to Pt with equivalent roughness. PtZn showed an enhancement indicating that Zn modification had impact on improving the ORR catalytic activity. From our experimental data, it could also be suggested that the PtZn surface becomes Zn-rich after cycling the potential in an oxygen saturated acidic solution. (C) 2008 Elsevier Ltd. All rights reserved.