We have recently used INS to determine how a hydrogen pre-treatment stage of an iron based Fischer-Tropsch catalyst may modify catalytic performance. That work established how critical the form of the catalyst activation procedure is in determining the sustainability of desired catalytic turnover. This new study builds on those findings and considers how a certain catalyst promoter (potassium) can modify the chemical form of the hydrocarbonaceous overlayer that is thought to control selectivity. The work is novel and will provide new information that is not readily obtainable by any other technique.