Electrochemical stability of electrodes based on C and Pt/C materials is studied in aqueous solution by means of the membrane inlet mass spectrometry technique (MIMS). This technique is based on mass spectrometry detection using a semipermeable membrane for the inlet of gases dissolved in liquid solutions. MIMS is applied to the study of gas evolving reactions taking place at electrodes made from platinum nanoparticles supported on carbon (Pt/C), and immersed in 0.5 M H2SO4 solution. Potentiodynamic experiments under MIMS detection show the formation of carbon oxidation products like CO2, CO and HCOOH. The oxidation of carbon is accelerated in the presence of Pt eletrocatalyst. Instability of a Pt/C electrode starts at 0.35 V-NHE, close to the C/CO2 thermodynamic potential, reflected by the detection of CO2 evolution. Partial oxygen reduction to hydrogen peroxide produces carbon corrosion at even lower potentials due to chemical reactivity. The corrosion rate is in the order of 10(-9) A cm(C)(-2) (related to total carbon area) at room temperature. The results are of interest for stability studies on proton exchange membrane fuel cell (PEMFC) electrodes. (c) 2006 Elsevier B.V. All rights reserved.