Gasification of low-rank coals in supercritical water was carried out in an autoclave by heating temperature to 690 degreesC and elevating autogenous pressure to 30 MPa. An attempt was made to investigate the catalytic effect of calcium hydroxide on the gasification. It was found that the presence of Ca(OH)(2) facilitates the extraction of volatile matter from coal and the decomposition of the volatile matter to small molecule gases, with a decrease in the amount of residual char. The production of Ka is enhanced by the catalytic effect of Ca(OH)(2) both on the water-added degradation of the volatile matter and on the gasification of the coal char, despite a slow procession of the char gasification. Moreover, Ca(OH)2 serves to capture CO2 as CaCO3. After the slurry of Ca(OH)(2) and a Yallourn coal with the Ca/C molar ratio of 0.6 had been heated at 690 degreesC and 30 MPa, the yields of H-2 and CH4 were, respectively, 0.35 and 0.18 NL/g of coal (daf), with little gaseous CO2 and liquid products.