The isomer distribution experimentally found for the saturated‐hydrocarbon products (pentane to octane range) from a cobalt Fischer‐Tropsch catalyst have been deduced from probability considerations. With certain restrictions, it is assumed that the carbon skeleton is built up by addition to any terminal carbon atom (for which the a priori probability has a constant value a) or to any penultimate carbon atom (with a priori probability b). Values of a and b, determined from experimental data, led to deduced isomer concentrations in each molecular‐weight cut agreeing with experimental concentrations with an average deviation of 0.7 percent. Peculiarities in the observed isomer distribution were also reproduced.