Carbon isotopic composition was measured for products of the Fischer-Tropsch synthesis: catalytic reaction between CO and H-2 to produce CO, CO2, light hydrocarbons C1-C4 and "oil" fraction. Hydrogen isotopes were also measured in the oil fraction and the produced water. Experimental runs were conducted in the flow-through reactor at 260-310 degrees C and 30 bar using the synthesis gas composed of 5N(2) + 3H(2) + 2CO, on Fe-catalyst mixed with ZSM-5 synthetic zeolite. In the two of seven runs a Fe + Co-catalyst was used that gives a lower yield of unsaturated hydrocarbons in reaction products. The isotopic effects depended on the conversion of the carbon monoxide. Under steady-state conditions (CO conversion more than 90 parts per thousand) a strong kinetic fractionation was observed between CO and CO2 (similar to-10 parts per thousand) and CO and hydrocarbons (similar to+38 parts per thousand). At low conversion a clear "inverse" isotopic trend of the depletion in I3C of longer hydrocarbon chains was observed. On average, Delta(12) = delta C-13(CH4) - delta C-13(C2H6) correlates well with the CO conversion: the C2H6 is similar to 6 parts per thousand isotopically lighter than CH4 at low conversion and similar to 2 parts per thousand, heavier at steady-state regime. Under steady-state conditions there almost no difference was observed in the isotopic composition of methane and ethane and higher hydrocarbons. The chemical composition of light hydrocarbons in the products of flow-through, dynamic FTS is different from that found in the static FTS-type experiments with Fe-catalyst, but isotopic effects are similar. Our results suggest that the isotopic distribution of carbon found in so-called "abiogenic" hydrocarbons from some natural gases (delta Cl-13 > delta(13)C2 > delta(13)C3 >...) is somewhat similar to that at low conversion of CO, but do not resemble the distribution characteristic for the high conversion products, at least, on Fe-catalyst. Other processes (a simple mixing of two or more endmembers) or other P-T conditions of the carbon reduction could be responsible for the "inverse" isotopic trend found in meteorites and some natural gases. (C) 2007 Elsevier Ltd. All rights reserved.