Fischer-Tropsch synthesis (FTS) is a catalytic reaction, which involves the production of liquid hydrocarbon fuel from synthesis gas obtained from natural gas, biomass or coal via gasification and steam reforming. From an industrial perspective, both Co and Fe based catalysts have been applied. However, Co-based catalysts are preferred in FTS particularly for gas-to-liquid (GTL) processes as they have high activity, high selectivity to linear hydrocarbons and low activity for the unwanted water-gas shift reaction. However, Co-based catalysts are relatively expensive and deactivate in time. To make the FTS process economically more effective, a stable performance of the catalyst is required. Therefore, studying the catalyst deactivation is an important topic in the development of better industrial catalysts. Oxidation, sintering of active phase and deposition of oxygenated compounds are potential causes for deactivation. The possible role of oxygenates and their effect of catalyst deactivation, however, is less understood. With the aim to investigate the deposition of oxygenated compounds, particularly carboxylates, as hypothetical deactivation mechanism, operando characterisation techniques were adapted to monitor the chemical and physical properties and structure-activity relationship of the catalyst during the reaction. Operando Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Mössbauer emission spectroscopy setups were employed that can be operated at industrially relevant FTS conditions.@en