Modified Ni catalysts supported on alumina and reduced in CH4 have been investigated for the synthesis of C-4 hydrocarbons from CH4 and C3H6. Addition of K or P to the Ni/Al2O3 catalyst increased C-4 selectivity and C3H6 conversion. A maximum selectivity to the desired C-4 product of 9 mol% was obtained at 350 degrees C and 101 kPa with a feed gas composition of 90 mol% CH4/10 mol% C3H6, but the catalyst activity declined rapidly with time-on-stream. Large amounts of unreactive carbon were deposited on the catalyst surface following reduction in CH4 and reaction in CH4/C3H6. However, the relative amount of a much more reactive species identified from Temperature-Programmed-Surface-Reaction that was formed in the presence of CH4 and C3H6, is shown to correlate with the catalyst C-4 yield. Both the C-4 yield and the relative amount of this low temperature carbonaceous species increased in the order Ni < Ni/P < Ni/K.