The phase transformation in a mixture of hydrated NaBiO₃ · nH₂O and dehydrated NaBiO₃ phase is studied during heating. The results indicate that generally the transformation occurs in two stages. At the first stage the hydrated NaBiO₃ · nH₂O phase converts to an anhydrous defective intermediate phase with the ilmenite-type structure similar to the initial dehydrated NaBiO₃, but with increased lattice parameter c. According to nuclear magnetic resonance (NMR) the intermediate phase is associated with additional inequivalent Na. In terms of the Na–O interatomic distances the intermediate phase is closer to the hydrated phase, but in terms of the symmetry of Na coordination polyhedron it is already closer to the dehydrated one. The Na of the intermediate phase still remains in the sample after the complete dehydration and at the second stage there is a gradual recrystallization of the intermediate phase towards the initial NaBiO₃ phase.