The relationship between the structural properties, such as the size and the shape, of a catalytic nanoparticle and its reactivity is a key concept in (electro) catalysis. Current understanding of this relationship is mainly derived from studies involving large ensembles of nanoparticles (NPs). However, the results necessarily reflect the average catalytic behavior of an ensemble, even though the properties of individual particles may vary widely. Here, we demonstrate a novel approach using scanning electrochemical cell microscopy (SECCM) to locate and map the reactivity of individual NPs within an electrocatalytic ensemble, consisting of platinum NPs supported on a single carbon nanotube. Significantly, our studies show that subtle variations in the morphology of NPs lead to dramatic changes in (potential-dependent) reactivity, which has important implications for the design and assessment of NP catalysts. The instrumental approach described is general and opens up new avenues of research in functional imaging, nanoscale electron transfer, and catalysis.