Carbon number distributions of Fischer-Tropsch products on iron and cobalt catalysts show deviations from the ideal Anderson-Schulz-Flory (ASF) distribution. For products obtained on cobalt catalysts these deviations are traced back by many authors to re-adsorption and incorporation of 1-alkenes followed by subsequent chain growth. In the present work, it could be shown by means of model calculations and based on experiments with co-feeding of ethene and 1-alkenes that such subsequent chain growth cannot be regarded as the main reason of observed deviations of the carbon number distribution from the ideal ASF distribution. The co-feeding experiments suggest that these deviations are the consequence of two different mechanisms of chain growth causing a superposition of two ASF distributions. Consequently, the carbon number distributions are represented by this superposition. In order to describe distributions as a function of reaction conditions the model parameter, the growth probabilities alpha(1) and alpha(2) as well as mu(1), the fraction of distribution (alpha(1)) are presented as function of the partial pressures of hydrogen and carbon monoxide. Finally, the typical model parameters of products formed on cobalt and iron are compared.