Abstract Phase formation of sodium bismuth titanate (Na 0.5 Bi 0.5 TiO 3 or NBT ) and its solid solution with barium titanate (BaTiO 3 or BT ) during the calcination process is studied using in situ high‐temperature diffraction. The reactant powders were mixed and heated to 1000°C, while X‐ray diffraction patterns were recorded continuously. Phase evolutions from starting materials to final perovskite products are observed, and different transient phases are identified. The formation mechanism of NBT and NBT – x BT perovskite structures is discussed, and a reaction sequence is suggested based on the observations. The in situ study leads to a new processing approach, which is the use of nano‐TiO 2 , and gives insights to the particle size effect for solid‐state synthesis products. It was found that the use of nano‐TiO 2 as reactant powder accelerates the synthesis process, decreases the formation of transient phases, and helps to obtain phase‐pure products using a lower thermal budget.