Fischer–Tropsch synthesis (FTS) using H2/CO/CO2 syngas mixtures over cobalt and iron based catalysts was carried out in a fixed-bed reactor. CO2 rich feeds produce products that are mostly light hydrocarbons with higher molar paraffin to olefin (P/O) ratio, whereas CO rich feeds shift the product composition to an FT type product with higher olefin product selectivity over both iron and cobalt based catalysts. Although the P/O ratio for FTS is strongly dependent on the operating conditions, the experimental evidence shows that the linear relationship between P(n+1)/O(n+1) and P(n)/O(n) holds for a large number of experiments. It is also shown to be independent of the type of the reactor, the composition of the syngas, reaction conditions and the kind of catalyst. Two features about the ratio of ξ = [P(n+1)/O(n+1)]/[P(n)/O(n)] for the FT products have been identified: (1) with n > 2, the experimental values of ξn>2 are higher than 1, fairly constant and independent of chain length n; (2) with chain length n = 2, the ratio of P3/O3 to P2/O2 (ξn=2) is significantly different, and shows that ξn=2 ≪ ξn>2. An equilibrium hypothesis is considered in an attempt to explain this experimental phenomenon.