Boron-doped diamond (BDD) electrodes have been modified by hydrous iridium oxide (IrO x ) electrodeposition, and depending on the deposition conditions, either highly dispersed iridium oxide nanoparticles or continuous oxide films were obtained. The electrochemical characteristics of IrO x on BDD compare well with those obtained on other substrate materials. The proposed method allows excellent control of the deposited amount, and high reproducibility of the electrochemical performance. Electrodes obtained by the deposition of a very low amount of IrO x (ca. 2 nmol cm−2) on the diamond surface exhibited an excellent analytical performance for hydrogen peroxide amperometric detection, that compares favorably with those obtained by using other substrate materials. Furthermore, these electrodes enabled H2O2 detection in neutral media, which is an advantage compared to sensors obtained by iridium oxide deposition on other substrates. The detection limit for H2O2 was ca. 10 times lower than that of commercially available platinum bulk electrodes. By proper selection of the diamond substrate structure, continuous IrO x layers of different thickness were formed. The IrO x /BDD electrodes have also exhibited excellent pH-sensing properties in a wide pH range, and high stability of the potentiometric response. These studies indicated that, by proper selection of the diamond substrate structure, robust IrO x layers could be prepared for use as H2O2 detection and pH sensors.