The direct determination of the localized, nanoscale structures which exist at a working catalyst surface contributes to the identification of the elemental building blocks from which its catalytic behavior is derived. Yet, such detailed information has been largely unavailable, especially in the high pressure regime (>1 bar) relevant for most heterogeneously catalyzed reactions. Here, we describe how scanning tunneling microscopy can be used to detect changes in the morphology of a model cobalt catalyst surface following CO hydrogenation at high pressures. A dynamic surface exists under these conditions. Cobalt mass transport most likely occurs via an etch-regrowth process and leads to the formation of triangular shaped cobalt islands.