This paper focuses on the synthesis of iso-paraffin-rich hydrocarbons by Fischer-Tropsch synthesis (FTS) over silica gel supported Co catalyst (CO/SiO2). The basic concept is to isomerize and/or hydrocrack the primary FTS hydrocarbon products. A physical mixture consisting of a small amount of zeolite or Pd/zeolite mixed with CO/SiO2 enhanced the formation of C-4-C-10 iso-paraffins while suppressing the formation of higher molecular hydrocarbons, probably because of the selective cracking of these hydrocarbons on them. In separate experiments, a two-reactor system was used. The first reactor contained a physical mixture of CO/SiO2 and zeolite, and the second reactor contained zeolites or Pd-supported zeolites. The two-reactor system gave sharp C-number distribution within C-3-C-6 and iso-paraffins-rich products. The hydrocracking of n-octane and n-decane (model compound simulating products of the FTS reaction) over mixed catalysts composed of various compositions of Pd/SiO2 and ZSM-5 in the presence of gaseous hydrogen showed high and stable activity, and produced primarily iso-paraffin-rich hydrocarbons. The isomerization was favored for mixtures rich in Pd/SiO2. The role of Pd was thought to be the inlet of hydrogen spillover to the zeolite surface. (C) 2004 Elsevier B.V. All rights reserved.