Submonolayer dissolution of platinum (Pt) nanoparticles under potential cycling is specified by a channel flow double electrode system. Dissolution of Pt+ starts at 0.6 V (vs. standard hydrogen electrode) in an anodic scan. The amount of dissolved Pt2+ below 1.0 V was calculated using collector current (I-C) and was confirmed by ex-situ inductively coupled plasma mass spectrometry. The amount of Pt2+ and pt(4+) dissolved in different potential regions during potential cycling was quantified by I-C for the first time. A mechanism of Pt dissolution under potential cycling in 0.5 M H2SO4 was proposed, and an in-situ system capable of quantitatively monitoring submonolayer dissolution of Pt2+ and Pt4+ was established. (C) 2014 The Electrochemical Society. All rights reserved.