Reactivities and surface areas of coals which were gasified in steam at high temperatures were determined. Reactivities were measured using a TGA and CO₂ adsorption method was used to measure surface areas. The reactivities of coal with steam decrease monotonically as conversion increase while char-steam reactions show a maximum between 5 to 15% conversion. Initial reativity of coal-steam reaction is much higher than char-steam reaction. Surface areas also decrease as conversion increase. The reactivity change was attributed to variation in surface area.