The activity of K- and Al-promoted Fe/MgO catalyst for Fischer-Tropsch synthesis was measured under atmospheric pressure, with the aim to determine the promoter’s influence on the catalyst selectivity, particularly in terms of olefins and isomers of lower hydrocarbons. The results have been correlated to the data of a previous study on the same catalyst, made under elevated pressure and designed for the alcohol production. It has been concluded that the promoting effect of K should be attributed to a geometric influence rather than to electronic promotion. The influence of Al promoter manifests itself like the "skin effect", resulting in suppression of adsorption of both CO and H-2.