Dissolution behaviour of Ru and Ru-oxide nanoparticles in acidic media was studied for the first time using highly sensitive concentration in-situ measurements with inductively coupled plasma mass spectrometry (ICP-MS). On-line time and potential resolved electrochemical dissolution profiles reveal novel corrosion features (signals) in the potential window from 0 V till ca. 1.4 V, where known severe dissolution due to oxygen evolution reaction (OER) takes place. Most of the features follow the thermodynamic changes of Ru oxidation/reduction state which consequently trigger the so-called transient dissolution. As-synthesised Ru sample exhibits an order of magnitude higher dissolution rate than electrochemically oxidised amorphous Ru sample. The latter, in turn, dissolves about ten times faster than rutile RuO2. Observed OER activity is in inverse relationship with measured dissolution. Disagreement is found with the general assumption that the onset of OER should coincide with the onset of Ru dissolutio