By using electrodeposited Pt nanoparticle electrodes, we clarify the relationship between the amount of dissolved Pt and the change in the electrochemical active surface area (ECA) of Pt due to potential cycling. Cyclic voltammetry of Pt was carried out in the pH range 0.4-4.0 in three different potential windows of 0.0-0.6, 0.0-1.0, and 0.0-1.4 V vs. RHE. The amount of dissolved Pt was almost negligible in the pH 3.0 solution; however, the ECA loss in this solution was almost similar to that observed in the pH 0.4 solution, in which the amount of dissolved Pt was the largest. The ECA loss was high up to the anodic potential limit of 1.0 V and almost remained constant when the potential was increased to the anodic potential limit of 1.4 V. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.065204jes] All rights reserved.