A unique feature of this study is the gasification of chars (in situ chars) in steam, starting from coals and proceeding uninterrupted, as opposed to a large number of previous studies, in which coals are first devolatilized in inert atmosphere, and the resulting chars (ex situ chars) are then gasified under conditions different from that of devolatilization. A large number of conversion and reactivity data were determined for both in situ and ex situ chars gasified in steam at 1000-1400 degrees C. The results obtained for bituminous coal, subbituminous coal and lignite indicate that the reactivities of the resulting in situ chars are up to six times higher than that of the corresponding ex situ chars. Further, the reactivities of all the in situ chars decrease monotonically as gasification proceeds, whereas maximum reactivities exist for ex situ chars at carbon conversions between 0.05 and O.15,. For all the in situ or the ex situ chars investigated in this work, the conversions at different gasification temperatures, when plotted against a dimensionless gasification time tau,＇unified＇ into a single characteristic curve with distinctly different shape for each type of char. The activation energies of the in situ chars obtained are in the range of 56-85 kJ/mol, while that of the ex situ chars are between 63 and 99 kJ/mol. In addition, the dependence of initial reactivity of the in situ char on steam partial pressure during gasification may be described by a Langmuir-type expression.