Use of a Fischer-Tropsch derived fuel in a fuel composition is disclosed, for the purpose of reducing catalyst degradation in a catalytically driven or catalyst containing system which is running on, or is to be run on, the composition or its products, wherein the Fischer-Tropsch derived fuel is used to reduce the level of silicon in the fuel composition, such as by reducing the concentration of silicon-containing antifoaming additive(s) in the fuel composition. It may also be used to reduce loss of efficiency of fuel atomization and/or combustion, and/or to reduce build up of silicon deposits, in a fuel consuming system which is running on, or is to be run on, the fuel composition.