The catalyst is crucial in Fischer-Tropsch synthesis (FTS). The activity and selectivity of the FT products could be affected during deactivation. The variation of the formation rate and selectivity to the light olefin and paraffin products with time on stream (TOS) over a cobalt catalyst has been investigated using a fixed bed reactor. The catalyst deactivated during the experiments, with the CO conversion dropping from 46.1% at 110 h TOS to 12.8% at 1200 h TOS. The selectivity to olefins increased with a drop in the CO conversion, whereas the selectivity to paraffin did not change much. A comparison of the rates before, during, and after deactivation showed that the olefin formation rate was fairly constant, and in some cases even increased, while the paraffin formation rate dropped, indicating that the deactivation affects mainly the paraffin formation rate. The results imply that when designing or operating a cobalt-based FTS process, the formation rates of olefin and paraffin should be considered separately.