The total product concentration and the paraffin/olefin ratio have been measured up to C-14 for Fischer-Tropsch synthesis on polycrystalline Co foils. The influences due to surface area, a wax coating, the H-2/CO ratio and flow velocity on concentration and selectivity have been determined. The paraffin/olefin ratio increases exponentially with chain length which is attributed to a chain-length-dependent olefin readsorption mechanism. The probability of readsorption depends on the heat of physisorption of the olefins on the catalyst as well as on their heat of dissolution in and their diffusivity through the product wax. All three factors predict an increase of the paraffin/olefin ratio with carbon number. Physisorption and dissolution are shown to cause a much stronger chain-length dependence than diffusion and will usually dominate.