Experiments were carried out in a fluidized bed reactor operated in a semi-batch mode using TiO2 or sodium trititanate to causticize either sodium carbonate or the sodium carbonate in kraft black liquor at temperatures between 800-825°C. Conversions of sodium carbonate of up to 88% were obtained, with the main product being sodium penta-titanate. Sodium hexatitanate and sodium tri-titanate are reaction intermediates formed when TiO2 is used. The BET surface area of the reacted solids decreased with increasing formation of sodium penta-titanate. Upon leaching of the final product, the surface area is regained. The high surface area of the leached material allows for good mixing with black liquor when the solids are recycled to the reactor. EDX analysis along the cross-section of TiO2 particles showed that sodium diffuses into the TiO2 particle, forming a very sharp front. This indicates that the direct causticization reaction proceeds according to the shrinking-core model, i.e. the reaction at the reaction front is fast compared to the diffusion of Na2CO3 into the reaction interface