Time- and potential-resolved electrochemical Pt dissolution from commercial Pt and prepared PtCu alloy nanoparticulate catalysts have been studied under potentiodynamic conditions in 0.1m HClO4 by using on-line inductively coupled plasma mass spectrometry (ICP-MS). For the first time the exact amount of dissolved Pt per cycle has been measured on real electrocatalysts. Results show clearly that Pt dissolution de- pends on the particle size: approximately seven times as much Pt is released into the solution from commercial 3 nm Pt parti- cles as from a commercial 30 nm Pt sample. The stability of our prepared PtCu electrocatalyst is higher than that of a com- mercial 3 nm electrocatalyst, which is, however, still slightly lower than that of a commercial 30 nm Pt electrocatalyst.