The mechanism of solid-state synthesis for Na_0.5Bi_0.5TiO₃ was investigated using X-ray powder diffraction, scanning electron microscopy and wavelength-dispersive X-ray spectroscopy. The results showed that a Na_0.5−xBi_0.5TiO_3−x/2 homogeneity region, with Na_0.5Bi_0.5TiO₃ and Na_0.47Bi_0.5TiO_3−δ as the end-members (0 ≤ x ≤ 0.03), exists in the Na₂O–Bi₂O₃–TiO₂ system. The formation of any member from the homogeneity region starts with the formation of the deficient end-member, i.e., Na_0.47Bi_0.5TiO_3−δ. However, the reaction continues toward a nominal composition when a sufficiently high temperature and/or a long firing time are used. The conversion of the latter reaction may be followed by the observation of the crystal symmetry, which changes from the cubic-like toward the rhombohedral as the reaction proceeds from Na_0.47Bi_0.5TiO_3−δ to Na_0.5Bi_0.5TiO₃. Differences in the dielectric, ferroelectric and piezoelectric properties were observed for the samples from the homogeneity region, and these can be related to the stoichiometry variation along the homogeneity region and thus to the variation in the vacancy concentration. The most striking difference is the change in the piezoelectric constant d₃₃, which increases from 82 pC N⁻¹ to 97 pC N⁻¹ as the composition of the ceramics changes from Na_0.5Bi_0.5TiO₃ to Na_0.47Bi_0.5TiO_3−δ.