It is shown that sampling along a catalyst bed combined with analysis by vapourphase chromatography is an informative technique for the investigation of catalytic reactions in a flow system. To illustrate the possibilities of the method, the decomposition of methanol, in the temperature range 163-210" C, is studied over a cobalt Fischer-Tropsch catalyst. The methanol decomposes to carbon monoxide and hydrogen, and the amounts of hydrocarbon and other synthesis products correspond to less than 1 % of the total reaction. The activation energy of the decompostion is 28.2 1.6 kcal/mole and the overall kinetics can be expressed by the equation dAdded hydrogen does not affect the rate of the decomposition, but the formation of methane is considerably enhanced by increase in the pressure of hydrogen. The significance of these results is discussed in relation to the nature of the initiating step in the Fischer-Tropsch synthesis.