A new product characterization model has been proposed for the iron-based low-temperature Fischer-Tropsch (Fe-LTFT) synthesis. The chain-length-dependent desorption model is based on the premise that the increase in chain-growth probability and decrease in the olefin/paraffin ratio with the carbon number in the Fe-LTFT synthesis is essentially a characteristic of the primary product spectrum. The model could successfully describe the olefin and paraffin distributions in the C3+ range. The ethylene/ethane ratio is overestimated by the model because of the high reactivity of ethylene for secondary hydrogenation. However, the total C-2 formation rate was predicted almost perfectly, while the methane formation rate was described adequately, using parameter values that were obtained from the C-3-C-10 product fraction. This is a true extrapolation, because the C-1 and C-2 data were not used at all for the estimation of the parameter values. This may be the first product characterization model that can successfully be extrapolated to the C-1 and C-2 components without introducing additional (unique) parameter values for these products.