Demineralized birch char co-loaded with nickel and calcium was hydrogasified to examine the catalytic behavior of the binary system at temperatures below 700 degrees C. The activity of nickel was determined by the amount of coexisting calcium, and it became a maximum at a given ratio of calcium to nickel. This can be explained by the two roles of calcium: (1) retardation of nickel sintering and (2) inhibition of the formation of catalytically active nickel species. Calcium was considered to interact with nickel to form a certain double oxide of calcium and nickel, whereby both actions occur. On the basis of this assumption, the function of calcium for nickel-and iron-catalyzed hydrogasifications of wood char is discussed.