Zeolite-containing Co catalysts for Fischer-Tropsch synthesis (FTS) were investigated in catalytic transformations of liquid hydrocarbons thus emulating secondary transformations of FTS products. Three different zeolites were employed, namely HBeta, HZSM-5 and HY. The conditions for catalytic testing were similar to those observed during FTS, i.e. typical temperatures in 170-260 degrees C range and the presence of water as a main byproduct of FTS. The results of hydrocarbon transformation investigation were analyzed vs performance of those same catalysts in FTS. It was established that all three investigated zeolites caused lower average molecular weight of the products of both FTS and liquid hydrocarbon transformations thus clearing the pore system of the catalyst from heavier hydrocarbons. It is remarkable that, despite the average molecular weight drop, the introduction of zeolites does not lead to increase in methane selectivity at Co catalysts in FTS. It was shown, in particular, that the composition of FTS-generated liquid hydrocarbons C5+ depends primarily on the properties of a particular zeolite emplyed in the Co catalyst, more precisely on the number and availability of Bronsted acid centers since these centers determine the activity of a catalyst in cracking and isomerization. Cracking was revealed to be decisive in elimination of wax-like heavy hydrocarbons while isomerization and oigomerization balanced both molecular weight distribution and group composition. Thus, zeolites can be instrumental in controlling composition of the products of Fischer-Tropsch synthesis produced over hybrid catalysts due to intensification of mass transfer.