A new simple characterization method was employed to investigate the formation of carbon deposits in reduced Co/Y zeolites. The method was based on temperature programmed surface reaction (TPSR) between adsorbed carbon deposits and hydrogen. Together with other characterization methods of FMR, TEM and H(2) chemisorption, CO hydrogenation reaction was performed on Co/Y zeolite catalysts, and the changes in CO hydrogenation activity and selectivity with metal distributions were also investigated. Especially the studies have discussions about the selectivity pattern of cobalt clusters within Y-zeolite cages and large cobalt metal aggregates on the external surface of the zeolites. The catalyst having most of cobalt metal clusters inside the zeolite cages showed much more selective synthesis of C(3) and C(4) products than the other catalysts. It is suggested that the lower hydrogenating ability induced the enhanced chain propagation and the chain growth was terminated by the surface sites of cobalt clusters. (C) 2010 Elsevier B.V. All rights reserved.