Conventional Fischer−Tropsch experiments were conducted to investigate the early stages (start-up) of the reaction of a TiO2-supported cobalt catalyst in a continuous stirred tank reactor and compared to the later stages. Thus, both short-term and long-term experiments were performed starting with fresh catalysts in each case. The experimental results showed that stirrer speed had an influence on the Fischer−Tropsch performance only in the short term. This suggested that under typical reaction conditions in a gas−solid system, long-term the Fischer−Tropsch reaction is not controlled by external mass transfer. After between about 30 and 80 h (depending on the temperature), large changes in reaction rate and product selectivity were observed, and these time-on-stream experiments showed that these changes were caused neither by the reaction conditions nor by the external mass transfer. Two possible explanations are proposed.