Fischer-Tropsch synthesis (FTS) over unsupported coprecipitated cobalt catalysts in n-decane in a batch slurry phase reactor by adding water vapor (H(2)O/CO = 0.12 in molar ratio) prior to reaction has been studied. The addition of water vapor exhibits a marked effect on the product selectivity. In the absence of water, the carbon number distribution of FT products follows the classical Anderson-Schulz-Flory (ASF) pattern resulting in a low selectivity (32%) in the desired C(10+) hydrocarbons. In contrast, with the promotion of water vapor, the formation of heavy products is appreciably increased up to 87.3% in C(10+) hydrocarbons so that the selectivity in the range of C(8)-C(30) increases obviously along with an increase in carbon number (n), which leads to a substantial deviation from ASF pattern. The effect of water is explained by suppressing secondary hydrogenation of 1-olefins and facilitating their readsorption and chain growth. (C) 2011 Elsevier B.V. All rights reserved.