Since Fischer-Tropsch (FT) synthesis is a chain growth reaction, its total product yield decreases exponentially with chain length forming a so-called Anderson-Schulz-Flory (ASF) distribution. Such a distribution is unselective toward middle distillates for all possible chain growth probabilities. Chain-length-dependent secondary reactions, however, cause deviations from the ASF-type distribution and can thus be used to improve the selectivity to the desired product range. To investigate secondary reactions we set out to study FT synthesis on flat model catalysts, a cobalt foil and cobalt particles on a SiO2 wafer, allowing a much better definition of the experiments than porous ones. On a Co foil olefin hydrogenation is the main chain-length-dependent secondary reaction, causing an exponential increase in the paraffin-to-olefin ratio with carbon number, but not resulting in a deviation from the ASF distribution. On Co/SiO2 model catalysts chain-length-dependent reinsertion of alpha-olefins into the chain growth process is the main secondary reaction, causing an increase of the growth probability with chain length. To a lesser extent, hydrogenolysis also plays a role, shortening long hydrocarbons by successive demethylation. On Co/SiO2 the interplay of chain-length-dependent reinsertion and hydrogenolysis results in sigmoid product distributions with a high selectivity to middle distillates. These product distributions can be fitted with a simple model in which a chain growth reaction is combined with chain-length-dependent secondary hydrogenation, reinsertion, and hydrogenolysis.