The reactivity of brown coal chars for low temperature steam gasification and the chemical forms and distribution of the naturally occurring calcium species between the bulk and the surface of chars and the evolution on carbonization and steam gasification were studied in this paper. The char surface was shown to be significantly enriched with calcium compared to bulk matter. Aragonite-like species were identified on the surface before char gasification, however they readily converted into the calcite-like ones under gasification conditions. Both forms of calcium carbonate species occurred in the nano-sized nucleus, neither of which was detected by XRD. The conclusion was made that these were highly dispersed calcite-like surface species derived from the aragonite-like ones under steam gasification conditions, which would be responsible for the catalysis of char gasification. (C) 2015 Elsevier Ltd. All rights reserved.