The disclosed invention relates to a process for converting a reactant composition comprising H 2 and Co to a product comprising at least one aliphatic hydrocarbon having at least about 5 carbon atoms, the process comprising: flowing the reactant composition through a microchannel reactor in contact with a Fischer-Tropsch catalyst to convert the reactant composition to the product, the microchannel reactor comprising a plurality of process microchannels containing the catalyst; transferring heat from the process microchannels to a heat exchanger; and removing the product from the microchannel reactor; the process producing at least about 0.5 gram of aliphatic hydrocarbon having at least about 5 carbon atoms per gram of catalyst per hour; the selectivity to methane in the product being less than about 25%.