The mechanism of carbon formation on nickel autothermal steam reforming catalysts has been studied by temperature-programming, thermogravimetric and electron microscopic techniques. Temperature programmed surface reaction (TPSR) studies of carbon deposited on nickel reforming catalysts by the decomposition of ethylene exhibit seven forms of carbon that are distinguished by their characteristic reactivity with H/sub 2/ and 3.0 vol% H/sub 2/O/He. The relative population of the different carbon states depends primarily on the temperature during deposition. The reactivity of the carbon states are not altered by exposure to steam in C/sub 2/H/sub 4/-H/sub 2/O mixtures, but the amount of carbon deposited decreases to zero as the H/sub 2/O/C increases past a critical ratio. Critical steam-to-carbon ratios have been measured in the presence of C/sub 2/H/sub 4/-H/sub 2/O mixtures so as to establish the carbon formation boundary for several alumina-supported nickel, Ni-Rh, and Ni-Ir alloy catalysts. The critical H/sub 2/O/C ratio measured with a gravimetric balance decreased from approx. 30 at 773/sup 0/K to approx. 1 at 1073/sup 0/K for all the catalysts studied. The very rapid rate of C/sub 2/H/sub 4/ reforming over this temperature range caused mass transport gradients, which in turn lowered the apparent critical H/sub 2/O/C ratios. Carbon deposited on a 17 wt% Ni/Al/sub 2/O/sub 3/ catalyst at 875/sup 0/K and 1073/sup 0/K was examined by transmission (TEM) and scanning (SEM) electron microscopy. Results are reported and discussed.