Product accumulation in Fischer-Tropsch synthesis (FTS) occurs not only in large continuously stirred tank reactors, but also in small fixed bed reactors. Thus, in order to obtain the correct product distribution data and paraffin to olefin ratio for a FTS reaction test, it is necessary to eliminate the accumulation effect even for reactions conducted in fixed bed reactors. This study shows that the D-2/H-2 switching method is a way to obtain the correct data for the FTS reactions. In a Co catalyzed FTS reaction, the alpha value determined by the D-2/H-2 switching method was close to 0.88 from C-9 to C-20. After accumulation and secondary hydrogenation corrections, the paraffin to olefin ratio for the hydrocarbons produced by this reaction was found to increase with increasing molecular size by a rate of e(0.15n), in which n is the number of carbon atoms. This is much smaller than the values obtained by the conventional method. (c) 2007 Elsevier B.V. All rights reserved.