The introduction of sorbents into the gasifier during in-situ desulfurization processes using calcium-based sorbents leads to the occurrence of some reactions, mainly between the calcium (Ca) in the sorbents and some minerals contained in the ash, especially iron (Fe). These reactions, which are between solid phases (solid-solid), affect the absorption ability of the sorbents. The occurrence of solid-solid reactions, however, depends on a number of factors. In this paper, the effect of sorbent particle sizes on the occurrence of solid-solid reactions in desulfurization processes has been investigated using a laboratory-scale fixed bed reactor. Mineral compositions in the spent sorbent-ash mixture were analyzed by X-ray diffraction, and the elemental distribution within individual particles was determined by the electron probe micro analysis (EPMA). Results show that the occurrence of solid-solid reactions increases with decreasing particle size, which is accompanied with the reduction of the sulfur absorption ability of a sorbent. Increasing the iron concentration in the coal ash led to the occurrence of more solid-solid reactions, suggesting that in high-iron-content coals, the effects of solid-solid reactions may be more serious. Despite the fact that iron is the major player in the occurrence of solid-solid reactions, other minerals present in the ash seem to play a catalytic role.