Based on the picture of Fischer-Tropsch synthesis as an ideal polymerization reaction it is easily noticeable that primarily formed product olefins can undergo secondary reactions, which generally lead to carbon number dependencies of certain olefin reaction pathways and thereby to modifications of product distributions. To obtain more insight into the role of secondary olefin reactions during Fischer-Tropsch synthesis and the reason for their chain length dependence, extensive studies on their extent and selectivity and particularly their chain length dependence at different reaction conditions (variation of CO partial pressure and reaction temperature) were performed by co-feeding alpha-olefins (C-2-C-11) during FT synthesis with cobalt catalysts in a gradientless slurry reactor. Conversion of the added olefins revealed a strong carbon number dependence exhibiting a sharp minimum at C-3. The observed chain length dependencies of individual reaction pathways (hydrogenation and incorporation) are in agreement with the selectivities observed in the base case experiments without olefin addition and mainly due to carbon number dependent solubility of the olefins in the liquid reaction product.