Minerals' attachment to calcium-based sorbent particles during in situ desulfurization of coal gasification process has been investigated. Results show that iron (Fe) from coal tends to attach itself to calcium (Ca) as a result of solid-solid reactions occurring during the process, and hence, forming complex compounds, e.g. calcium iron oxide (CaFe3O5) and ilvite (CaFe3(SiO4)(2)OH). Other alkali metals included in the ash do not show much of this tendency. The occurrence of solid-solid reactions reduces the efficiency of the sorbents due to the reduction of effective absorption area and also leads to the agglomeration of particles. Results also show that these reactions tend to occur in very fine particles at localized locations. This suggests that a proper choice of sorbent particle sizes may reduce their effect.