Direct dry sorbent injection is a relatively simple and low-cost process for the capture of SO2 emissions, particularly for in-bed capture during fluidized bed combustion. SO2 capture by limestone may be accomplished by either direct or indirect sulfation, depending on gas-phase environment and operating conditions. In this paper, a combination of thermogravimetric analysis (TGA) and a bench-scale bubbling fluidized bed reactor were used to study sulfation, calcination and carbonation behavior of limestone at different CO2 concentrations and various temperatures to identify the relative contributions of indirect and direct sulfation reactions under air- or oxy-fired fluidized bed combustion conditions. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy analyses were also used to help identify product layer microstructure and composition. A significant temperature dependence on the sulfation of limestone was seen under oxy-fired conditions (when direct sulfation dominates); however, limited temperature dependence was seen for air-fired conditions (where indirect sulfation dominates). A distinct CaSO4 product layer is seen in SEM images for the air-firing condition that can significantly inhibit gas transport into internal pores. No such distinct product layer was seen for oxy-firing conditions. For the size range of limestone used in this study (600 to 1000 mu m), indirect sulfation (dominant under air-fired conditions) appeared to be strongly limited by diffusion, whereas direct sulfation (dominant under oxy-fired conditions) was mainly under kinetic control or a mixture of kinetic and diffusion control.