We investigated the electrochemical detection of single iridium oxide nanoparticle (IrOx NP) collisions at the NaBH4-treated Pt ultramicroelectrode (UME) in a scanning electrochemical microscope (SECM) over an insulating surface. The NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where it does not take place on the Pt UME. These collisions occurred stochastically, resulting in a transient response ("blip") for each collision. The frequency of the collisions is proportional to the flux of NPs to the UME tip, and thus equivalent to the SECM current. A plot of collision frequency versus distance followed the theoretical approach curve behavior for negative feedback for a high concentration of mediator, demonstrating that the collisions were diffusion-controlled and that single-particle measurements of mass transport are equivalent to ensemble ones. When the SECM was operated with a Pt substrate at the same potential as the tip, the behavior followed that expected of the shielding mode. These studies and additional ones result in a model where the IrOx NP collision on the Pt UME is adsorptive, with oxygen produced by the catalyzed water oxidation causing a current decay. This results in a blip current response, with the current decay diminished in the presence of the oxygen scavenger, sulfite ion. Random walk and theoretical bulk simulations agreed with the proposed mechanism of IrOx NP collision, adsorption, and subsequent deactivation.