The factors which affect the loss of activity of a nickel alumina catalyst for the gasification of sulfur-free light hydrocarbons with steam at temperatures below 500 °C have been examined. These are sintering or loss of active surface area due to heat treatment in steam, and poisoning of active sites by a much slower parallel reaction to gasification involving constituents of the hydrocarbon feedstock. The second of these modes of activity loss is much more important at temperatures between 400 and 500 °C and normal operating pressures around 25 atm. This second mode of poisoning is independent of total pressure but rises rapidly with the boiling point of the hydrocarbon feedstock and the hydrocarbon steam ratio supplied. Increases in temperature favor the gasification reaction at the expense of poisoning. With finely divided catalyst particles, the rate of the gasification reaction is chemically controlled and zero order with respect to steam and hydrocarbon. With larger particles (0.3 cm diam) the rate is controlled by gaseous diffusion in the pores of the catalyst and, in line with this, the reaction rate becomes proportional to the particle size, the reaction order rises, and the activation energy falls. Rate control is probably by Knudsen diffusion in the micropores, though bulk diffusion in the macropores cannot be ruled out. The poisoning rate falls sharply at first on entering the region of diffusional control as the particle diameter is increased, but ultimately becomes independent of size. An attempt is made to interpret this phenomenon in terms of the effects of diffusional rate control on steam hydrocarbon ratio and temperature.