Metallic iridium and ruthenium as well as their oxides are among the most active oxygen evolution (OER) electrocatalysts in acidic media, and are also of interest for the catalysis of the hydrogen evolution (HER). The stability of these materials under different operating conditions is, however, still not fully understood. In the current work, activity and stability of well-defined Ru, RuO2, Ir, and IrO2 thin film electrodes are evaluated in acidic and alkaline electrolytes using an electrochemical scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS). Identical experimental protocols are intentionally employed for all electrodes and electrolytes, to obtain unambiguous and comparable information on intrinsic activity and stability of the electrodes. It is found that independent of the electrolyte, OER activity decreases as Ru >Ir approximate to RuO2 >IrO2, while dissolution increases as IrO2 << RuO2< Ir << Ru. Moreover, dissolution of these metals in both solutions is 2-3 orders of magnitude higher compared to their respective oxides, and dissolution is generally more intense in alkaline solutions. Similarly to the OER, metallic electrodes are more active catalysts for HER. They, however, suffer from dissolution during native oxide reduction, while Ir02 and Ru02 do not exhibit significant dissolution. The obtained results on activity and stability of the electrodes are discussed in light of their potential applications, i.e. water electrolysers or fuel cells. (C) 2015 Elsevier B.V. All rights reserved.