Temperature-driven and composition-driven phase transformations in a mixed A-site Pb-free solid solution series of (100−x)Na0.5Bi0.5TiO3–xSrTiO3 (0≤x≤40) were investigated by means of x-ray diffraction, dielectric, and Raman scattering techniques over a temperature range of 20–550 °C. The phonon anomalies observed in the room temperature Raman spectra with increasing Sr concentration revealed a morphotropic phase boundary (MPB) at x∼20. The distinct and discontinuous changes in dielectric properties at x∼20 confirmed the existence of MPB. On heating, the Raman spectral features for different compositions displayed substantial changes at temperatures almost identical to the critical temperatures determined by dielectric measurement, such as ferroelectric–antiferroelectric (Td), rhombohedral–tetragonal (TR-T), and antiferroelectric–paraelectric (Tm) phase transition temperature. However, the two phonon anomalies observed at higher temperatures (∼400 and 500 °C for x=10) suggested the coexistence of rhombohedral–tetragonal and tetragonal–cubic phases, respectively. The transition temperatures determined by the Raman and dielectric measurements gradually decreased as Sr concentration increased. The system showed strong relaxor behavior at x≥20 and the dielectric constant (ε) at room temperature was enhanced significantly from ∼500 (x=0) to ∼3800 (x=40) at 1 kHz.