The effects of the constituents of mineral matter in brown coals from different deposits of Kansk-Achinsk, Lenaand from Yallourn Basins on the structural parameters and steam gasification reactivities of respective coal chars at moderate temperature and at low and high pressure were studied in this paper. The data on how the preliminary decationization with diluted hydrochloric, acetic and sulphuric acids affect char gasification reactivities are presented. The importance of surface area and crystallinity of chars and the presence of naturally occurring metals on gasification reactivity is considered. Quantitative correlations between the calcium contents and the extents of gasification are revealed. The gasification results obtained in a flow reactor with steam stream and in an autoclave reactor at high pressure of gaseous products are compared. The catalytic effect of dispersed calcium oxide-carbonate particles produced from the naturally occurring calcium containing carboxylates was shown to be a key factor for char gasification reactivity, the effect in the flow reactor being much larger as compared to that in the autoclave reactor. This was mainly related to different forms of catalytically active calcium species and to the composition of the gaseous reaction mixture.