Iron Fischer-Tropsch catalysts with varying amounts of K (0, 2, 4, 6, 10 wt%) were utilized. Except possibly for the 10 wt% catalyst, the amount of the iso-alkanes produced was essentially constant for carbon numbers from 4 to 23. Iso-alkanes accounted for about 13-15 % of the hydrocarbon products that were produced. The 2- and 3-methyl isomers were present at about two times the amount of the other methyl isomers for each carbon number. Mechanisms to account for the branching were considered but a satisfactory one could not be found.