Abstract The thermal stability and nanoscale structural evolution at elevated temperatures of a sputter deposited W-Ti alloy thin film were studied by a combination of ex situ and in situ techniques. XRD, FIB, SEM-EDX and STEM-EDX were used to characterise the film annealed ex situ in vacuum at 1373 K for 48 h. In situ TEM heating experiments were conducted at various temperatures up to 923 K to capture transitional phenomena occurring in the alloy upon heating and cooling. At a microscopic level, the alloy annealed at 1373 K for 48 h transformed from a single-phase β-(WTi) solid solution into a two-phase alloy consisting of Ti-rich grains in equilibrium with Ti-depleted β-(WTi) solid solution grains. In situ TEM observations revealed initial Ti segregations along columnar grain boundaries at T ∼ 423–573 K, followed by Ti-rich clusters formation in the grains interior at T ∼ 573–773 K. The microstructure observed at 923 K remained stable upon cooling to room temperature and consisted of Ti-rich segregations along the columnar grain boundaries and of alternate Ti-rich and Ti-depleted nanoscale domains in the grains interior, which formed a stable dual-phase nanocrystalline structure.