It is known that several factors influence the ability of calcium to catalyse the steam-activated gasification of chars derived from low rank coals. One of the must important is that calcium which is ion exchanged on to the parent coal is more likely to become catalytically active in the char than calcium which is present in the coal in inorganic crystalline form. The findings reported here show that a great deal of catalytic activity is realised when only a small fraction of the available ion exchangable sites in the coal contain calcium and that further addition leads to comparatively small rate increases. It is also found that the calcium influences the product gas mixture through driving the water gas shift reaction toward equilibrium. In order to do this, it is necessary for the initial products of gasification to remain near the char surface for some time. When the initial products are rapidly swept away from the surface, the ability of calcium to influence product gas distribution is severely inhibited.