IrOx electrodes suffer from large potential drift in solutions in the presence of strong reductive anions such as S2-, I-, etc. resulting in the inaccuracy of pH measurement. Our homemade IrOx electrode developed by cycling heat-treatment and quenching process for three times with excellent response property showed the same problem. In-situ/ex-situ XAFS study showed the composition of the IrOx film changes in strong reductive solution. At the meantime, XPS survey of the IrOx electrode also verified the composition change of Ir4+/Ir3+ before and after its immersion in these solutions. In order to solve the above problem, the porous-structured IrOx electrodes weremodified by being dipped into Nafion solution for 1 similar to 3 times. Surface and cross section characterizations, EIS tests as well as the pH detection property evaluations were conducted to identify the modification effect and determine a better modification process. Nafion modification for just once is suggested to prevent the electrode from potential drift in solutions containing strong reductive anions. Finally, the pH response mechanism of the modified IrOx electrode in reductive solutions was discussed. (C) The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License.