Carbon isotope transients ((CO)-C-12 --> (CO)-C-13) at reaction steady state were used to correlate the in situ surface inventories of active carbon species with product selectivity variations in the Fischer-Tropsch synthesis. The measurements were conducted on an unsupported cobalt catalyst at a variety of elevated pressure conditions (syngas pressures from 3 to 15 atm, H-2:CO ratios from 1 to 4, temperatures from 190 to 228 degreesC, and cofed water vapor pressures from 0 to 8 atm). The in situ adsorbed CO inventory is saturated and insensitive to the changes in reaction conditions, except for a small decrease in the presence of elevated water pressures. A large amount of active carbon intermediates is present as a monomeric species on the catalyst, and this amount varies widely with reaction conditions (adsorbed C*/CO values from 0.3 to 0.8). Changes in active carbon inventory cause proportional changes in the polymerization probability at all carbon numbers. For the most part, carbon monoxide and water influence the polymerization probability indirectly by causing changes in the monomer C* amounts. Hydrogen and temperature do not cause significant changes in monomeric carbon, but influence the rate coefficients directly. The influences of gas composition on the rate coefficients provide useful information regarding the site requirements for chain growth. (C) 2004 Elsevier Inc. All rights reserved.