In this paper, we present a simple, relatively inexpensive, and reproducible method for the construction of iridium oxide (IrOx)-pH sensitive ultramicroelectrodes (UMEs). In this method, the pH sensing layer is based on an IrOx film formed by potential cycling of iridium microparticles (diameter < 10 mu m) in 0.5 M H2SO4 at 3.0 V/s. The IrOx-pH microsensors prepared this way are chemically stable, uniform, and exhibit rapid and linear response to H+ over the range 2-12 pH with a super-Nernstian slope (-64 +/- 7 mV/pH) at room temperature. The pH response is reproducible and remains stable over a period of 2 months. Coexistence of redox ions such as Fe(CN)(3-)(6) and Ru(NH3)(3+)(6) has little influence on the determination, however Ni2+ does produce an apparent pH shift. The application of these pHsensitive UMEs is demonstrated in imaging the pH changes due to the oxygen reduction reaction at the surface of a cast iron sample. (C) 2007 Elsevier B.V. All rights reserved.