[en] Using density-functional ab initio theoretical techniques, we study (Ga_1−xIn_x)₂O₃ in both its equilibrium structures (monoclinic $\mathrm{\beta <!--\; ??\; -->}$ and bixbyite) and over the whole range of composition. We establish that the alloy exhibits a large and temperature-independent miscibility gap. On the low-x side, the favored phase is isostructural with $\mathrm{\beta <!--\; ??\; -->}$-Ga₂O₃; on the high-x side, it is isostructural with bixbyite In₂O₃. The miscibility gap opens between approximately 15% and 55% In content for the bixbyite alloy grown epitaxially on In₂O₃, and 15% and 85% In content for the free-standing bixbyite alloy. The gap, volume and band offsets to the parent compound also exhibit anomalies as function of x. Specifically, the offsets in epitaxial conditions are predominantly type-B staggered, but have opposite signs in the two end-of-range phases. (paper)