This study outlines the methodology to electrodeposit well-defined iridium nanospheres onto glassy carbon (GC) electrodes at room temperature, using a square wave potential (SWP) technique. Similar work has recently been carried out for other noble metal nanoparticles, in particular platinum and palladium. In this study, particle size and Ir loading can be controlled by altering two key parameters in the electrodeposition process; the nucleation potential and the growth duration. Ir nanoparticles were characterized using cyclic voltammetry (CV) and scanning electron microscopy (SEM). It was found that the nucleation potential and time influence not only the nucleation density but also the particle size. In addition, the electrochemical activity of the particles towards carbon monoxide (CO) adlayer oxidation was investigated as a surface test reaction. The results showed that surface morphology of the deposited particles, which could be altered using potential cycling, plays a pivotal role in determining the activity of the particles. Particles that were treated by potential cycling showed a significantly lower overpotential, in contrast to particles of similar size that were untreated. This indicates an enhancement in electrocatalytic activity due to the enrichment of surface defects.