The electrochemical detection of hydrogen peroxide and nitrite, as important representatives of cytotoxic reactive oxygen/nitrogen (ROS, RNS) species released during cellular oxidative bursts, is reported in microfluidic devices. Platinum-black coated platinum (Pt/Pt-black) electrodes were microfabricated and optimized to achieve optimal detection performances. In parallel, oxidation mechanisms of hydrogen peroxide and nitrite at these microelectrodes were investigated and electrode responses were compared to theoretical predictions based on convective mass transport at microchannel electrodes. In both cases, the active surface area of Pt/Pt-black electrodes allowed to avoid inhibition effect leading to long term stability in contrast to bare Pt electrodes. Such highly sensitive Pt/Pt-black electrodes allowed almost five decades of concentration range and detection limits down to 10 nM, a range suitable for detection of ROS and RNS released by a few cells.