Supported nano-sized metal crystallites as catalysts in the Fischer-Tropsch synthesis have become a major research focus due to their high mass specific surface area and resulting lower cost. Such small supported cobalt crystallites have been reported to show a very different resistance with regard to deactivation compared to larger cobalt particles. The Fischer-Tropsch product water is reported to have a severe effect on the deactivation of cobalt-based Fischer-Tropsch catalysts. Compared to other water-induced deactivation mechanisms, hydrothermal sintering of cobalt nanoparticles is fairly well established in literature. A previously hypothesised interconnection between oxidation of cobalt nanoparticles and hydrothermal sintering has - for the first time - been captured in situ in the presented study. High concentrations of water induce oxidation of the cobalt nanoparticles increasing their mobility and resulting in crystallite growth via particle migration and coalescence whilst in the oxidised state. A well-defined model catalyst comprising highly dispersed cobalt nanoparticles on a relatively inert exfoliated graphite support in combination with an in situ magnetometer allowed for these observations, which resulted in irreversible deactivation of the catalyst. (C) 2019 Elsevier Inc. All rights reserved.