CFBC boilers firing high sulfur, low ash fuels can produce deposits consisting of almost pure CaSO4. Test work has shown that if sorbents or bed materials are sulfated for long times (several days or more), they develop compositions similar to those deposits. Sulfation increases with temperature from 650 degreesC to 900 degreesC, and strength increases continuously in the temperature range of 650 degreesC to 950 degreesC. When sulfation occurs under conditions where CaCO3 is stable, the overall strength of the deposit increases but the degree of sulfation diminishes. Finally, this work suggests that pellet tests using crushed and calcined sorbents can give misleading information and should be used with caution to study the phenomena described here.