A planar-type amperometric dual microsensor for simultaneous detection of nitric oxide and carbon monoxide is presented. The sensor consists of a dual platinum microdisk-based working electrode (WE) and a Ag/AgCl counter/reference electrode covered with an expanded poly-(tetrafluoroethylene) (Tetra-tex) gas-permeable membrane. The dual WE possesses two different platinized platinum disks (WE1 and WE2, 250 and 25 μm in diameter, respectively). The larger WE1 is further modified with electrochemical deposition of tin. Use of two sensing disks different in their size as well as in their surface modification produces apparently different sensitivity ratios of NO to CO at WE1 and at WE2 (∼2 and ∼10, respectively) that are induced by favorable CO oxidation on the surface of tin versus platinum. Anodic currents independently measured at WE1 and at WE2 are successfully converted to the concentrations of NO and CO in the co-presence of these gases using the differentiated sensitivities at each electrode. The sensor is evaluated in terms of its analytical performance: respectable linear dynamic range (sub nM to μM); low detection limit (∼1 nM for NO and <5 nM for CO); selectivity (over nitrite up to ∼1 mM); and sensitivity (sufficient for analyzing physiological levels of NO and CO). Using the NO/CO dual microsensor, real-time, simultaneous, direct, and quantitative measurements of NO and CO generated from living biological tissue (mouse, c57, kidney) surfaces, for the first time, are reported.