Metal nanoparticles are used as catalysts in a variety of important chemical reactions(1,2), and can have a range of different shapes(3-8), with facets and sites that differ in catalytic reactivity(1,2,9). To develop better catalysts it is necessary to determine where catalysis occurs on such nanoparticles and what structures are more reactive. Surface science experiments or theory can be used to predict the reactivity of surfaces with a known structure(1,2,10), and the reactivity of nanocatalysts can often be rationalized from a knowledge of their well-defined surface facets(3-5). Here, we show that a knowledge of the surface facets of a gold nanorod catalyst is insufficient to predict its reactivity, and we must also consider defects on the surface of the nanorod. We use super-resolution fluorescence microscopy to quantify the catalysis of the nanorods at a temporal resolution of a single catalytic reaction and a spatial resolution of similar to 40 nm. We find that within the same surface facets on the sides of a single nanorod, the reactivity is not constant and exhibits a gradient from the centre of the nanorod towards its two ends. Furthermore, the ratio of the reactivity at the ends of the nanorod to the reactivity at the sides varies significantly from nanorod to nanorod, even though they all have the same surface facets.