The use of Ca-based sorbents in circulating fluidized beds (CFB) allows the in-situ desulfurization in oxyfuel combustion processes. The sulfation process involves important changes in the sorbent morphology, which could vary depending on the operating conditions and be different to those observed in conventional air combustion. This work analyzes the morphological variations observed during limestone and dolomite sulfation at typical oxy-fuel combustion conditions (high CO2 concentration, higher temperatures than in air combustion) in CFB combustors (long reaction times). Sulfated samples prepared in a thermogravimetric analyzer were analyzed by Scanning Electron Microscope (SEM). The space limitations due to the higher molar volume of CaSO4 compared to CaO in the external surface of the particles make that the CaSO4 product layer trend to grow outwards to form a honeycomb-shaped structure. This structure appeared for limestone at both calcining and non-calcining conditions. A strong effect of the CaSO4 sintering phenomenon was observed at temperatures above 950 degrees C. Moreover, the honeycomb structure was never observed working with dolomite in spite of the high sulfation conversions reached with this sorbent. (C) 2015 Elsevier Ltd. All rights reserved.