We have demonstrated wide bandgap and composition range b-(AlxGa1x)2O3 thin films by employing thermal annealing of b-Ga2O3/ sapphire templates. With proper annealing conditions at 1000–1500 C, the b-Ga2O3 thin films transformed to the b-(AlxGa1x)2O3 thin films with different bandgaps and compositions due to the Al diffusion from sapphire. Meanwhile, the Ga atoms diffused into sapphire. The interdiffusion process caused an increased film thickness, which was enhanced in proportion to the annealing temperature. It was confirmed by secondary ion mass spectrometry (SIMS) and transmission electron microscopy. Thus, higher temperatures resulted in high Al contents in the b-(AlxGa1x)2O3 films. Also, the SIMS measurements show highly homogeneous Al contents throughout the b-(AlxGa1x)2O3 films annealed at 1200 C and above. Evaluated by x-ray diffraction (XRD), the Al content range of the samples is 0–0.81 for the b-Ga2O3 templates without annealing and with annealing up to 1500 C. Evaluated by UV-Vis spectroscopy, the optical bandgap range of the samples is 4.88–6.38 eV for the b-Ga2O3 templates without annealing and with annealing up to 1400 C, translating to the Al content range of 0–0.72. Moreover, the crystal quality of b-(AlxGa1x)2O3 improved as the Al composition became larger due to higher annealing temperatures. The proposed technique is promising for the preparation of b-(AlxGa1x)2O3 thin films without employing “direct-growth” techniques.