Steam gasification of 16 coals with different ranks and sulfur contents was carried out isothermally at 873-973 K in the presence of Ca(OH)(2) with a thermobalance. The catalyst at 5 wt % Ca promotes the gasification of all the coals. The calcium on low-rank coals giving moderate reactivities without catalyst shows large rate enhancement, which leads to complete gasification within 30 min at 973 K and lowers the gasification temperature by 110-150 K. The finely dispersed Ca species is formed on the chars after devolatilization of low-rank coals with Ca(OH)(2), and responsible for high catalytic activity. The calcium catalyst is less effective with high sulfur coals, and some of the sulfur evolved on coal devolatilization is captured by the calcium, but CaS loaded onto brown coal is still active in the gasification. Apparent activation energies are unchanged or slightly decreased by Ca(OH)(2) addition in mast-cases. The catalytic effect is discussed in terms of catalyst dispersion, sulfur poisoning, and inherent minerals.