The dependencies of hydrocarbon product distributions of iron and cobalt catalyzed Fischer-Tropsch synthesis on partial pressures of reactants have been studied. For cobalt catalysts particular attention has been focussed on the modification of distributions by secondary chain growth of readsorbed 1-alkenes while for iron catalysts secondary chain growth has been proved as negligible. The widely discussed concept of two superimposed Anderson-Schulz-Flory distributions has been applied for the representation of product distributions for both iron and cobalt catalysts. Based on l-alkene and ethene cofeeding experiments with cobalt catalysts and the promoter effect of alkali on iron catalysts the conclusion has been drawn that superimposed distributions with different chain growth probabilities are the result of different chain growth mechanisms. These results and the dependence of product distribution on the partial pressures of hydrogen and carbon monoxide have lead to the conjecture that products associated with the lower growth probability are formed by the well accepted CH2 insertion mechanism.