The promoter effect of alkali on Fischer-Tropsch iron catalysts causes an increased 1-alkene selectivity, a slightly increased reaction rate, an increased growth probability of hydrocarbon chains and also an increased resistance against oxidation of iron by the reaction product water. Experiments are presented which show that for cobalt catalysts alkali addition also leads to increased 1-alkene selectivity. However, the reaction rate is markedly reduced. The effect on the 1-alkene selectivity is without doubt due to increased adsorption strength of carbon monoxide causing an enhanced displacement of 1-alkenes while the propensity towards hydrogenation is hardly reduced. For iron catalysts the 1-alkene selectivity increases in the turn of Li, Na, K, Cs. With respect to the bimodal Anderson-Schulz-Flory (ASF) distribution the strong effect on the growth probability alpha(2) is independent of the nature of the alkali cation while the fraction f(2) of the distribution that is characterized by alpha(2) increases in the turn Li, Na, K, and Cs. These strong promoter effects are interpreted on the basis of a novel mechanism of Fischer-Tropsch synthesis whereby the alkali cation takes part in the catalytic circle. Finally an analogy of the promoter effect of alkali on iron for the ammonia synthesis and the Fischer-Tropsch synthesis is suggested. (C) 2008 Elsevier B.V. All rights reserved.