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Phase pure β-(Al x Ga 1−x ) 2 O 3 thin films are grown on (001) oriented β-Ga 2 O 3 substrates via metalorganic chemical vapor deposition. By systematically tuning the precursor molar flow rates, the epitaxial growth of coherently strained β-(Al x Ga 1−x ) 2 O 3 films is demonstrated with up to 25% Al compositions as evaluated by high resolution x-ray diffraction. The asymmetrical reciprocal space mapping confirms the growth of coherent β-(Al x Ga 1−x ) 2 O 3 films (x < 25%) on (001) β-Ga 2 O 3 substrates. However, the alloy inhomogeneity with local segregation of Al along the ([Formula: see text]) plane is observed from atomic resolution STEM imaging, resulting in wavy and inhomogeneous interfaces in the β-(Al x Ga 1−x ) 2 O 3 /β-Ga 2 O 3 superlattice structure. Room temperature Raman spectra of β-(Al x Ga 1−x ) 2 O 3 films show similar characteristics peaks as the (001) β-Ga 2 O 3 substrate without obvious Raman shifts for films with different Al compositions. Atom probe tomography was used to investigate the atomic level structural chemistry with increasing Al content in the β-(Al x Ga 1−x ) 2 O 3 films. A monotonous increase in chemical heterogeneity is observed from the in-plane Al/Ga distributions, which was further confirmed via statistical frequency distribution analysis. Although the films exhibit alloy fluctuations, n-type doping demonstrates good electrical properties for films with various Al compositions. The determined valence and conduction band offsets at β-(Al x Ga 1−x ) 2 O 3 /β-Ga 2 O 3 heterojunctions using x-ray photoelectron spectroscopy reveal the formation of type-II (staggered) band alignment.