PFBC technology is still under development. Further research into the capture of SO2 by limestone during PFBC is needed. The present work investigates the influence of periodically changing partial pressure of CO2 on the sulfation of limestone and also the free lime content present in residual products. Experiments were conducted in a fixed-bed reactor under conditions typical of PFBC, that is, limestone particles were alternately exposed to the conditions of calcination and recarbonation as the reaction of sulfation proceeded. It was found that the degree of sulfation achieved was considerably higher with periodically changing partial pressure of CO2 than with a steady high partial pressure of CO2. This was true even when the average value of the pressure during its periodical change exceeded the calcination equilibrium pressure. These findings may partly explain the high conversion of limestone obtained in PFBC units. Furthermore, the free lime content proved remarkably different for various cycle times of changing CO2 partial pressure and/or various periods of a cycle time elapsing in the calcining region. This indicates that the content of free lime in solid residues from PFBC boilers can vary considerably with respect to overall operating conditions.