<p>The paper presents the study of phase and structural behavior of pseudoternary compound (Ga_1-<i>x-y</i>In_<i>x</i>Al_<i>y</i>)₂O₃ with focus on the compositional cross-section where the <i>x</i>/<i>y&#xa0;</i>ratio is a fixed at 0.31/0.69. This specific ratio ensures that the average cation radius, equal to that of Ga³⁺ ions, remains unchanged. Through the combination of experimental XRD studies and DFT calculations, the stability region of the monoclinic phase within the Ga₂O₃–Al₂O₃–In₂O₃ ternary system was established. Detailed analysis of the crystal lattice parameters and unit cell volume of the monoclinic structure was carried out across a wide range of compositions. An empirical relationship was derived linking the monoclinic lattice parameters to the average ionic radius of the cations (Ga³⁺, Al³⁺, In³⁺) enabling prediction of lattice parameters in monoclinic (Ga_1-<i>x-y</i>In_<i>x</i>Al_<i>y</i>)₂O₃ solely from chemical composition. The experimental crystal structure studies and the electronic structure calculations suggest that in the monoclinic (Ga_1-<i>x-y</i>In_<i>x</i>Al_<i>y</i>)₂O₃ structure the tetrahedral positions of Ga1 atoms are preferentially occupied by Ga³⁺ and Al³⁺ cations, while the octahedral Ga2 sites accommodate a mixture of Ga³⁺, Al³⁺ and In³⁺ cations. Additionally, the presence of unidentified phase(s) was confirmed in the central region of the Ga₂O₃–Al₂O₃–In₂O₃ triangle. Comparison of the calculated optical absorption spectra and the Tauc-plots derived from the diffuse reflectance spectra indicate that the monoclinic (Ga_1−x−yIn_xAl_y)₂O₃ compounds have a direct band gap.</p>