Five single crystals of Y3Al5−xGaxO12 (0 ≤ x ≤ 5) garnet solid solutions with the compositions x = 0.0, 1.0, 2.0, 3.0 and 4.0 were synthesized using a flux method. The compositional dependence of the lattice constants of the garnet solid solutions shows a large deviation from Vegard's law. Investigation of the cation distributions of these garnets using single-crystal X-ray diffraction shows that Ga3+, which is larger than Al3+, preferentially occupies the tetrahedral (four-coordinate) site rather than the octahedral (six-coordinate) site. On the basis of the results obtained from structure refinements, geometric analyses of the polyhedral distortions were carried out. The results imply that the cation–cation repulsive force across the polyhedral shared edges decreases with increasing substitution of Ga3+. Moreover, the proportion of covalent bonding in the cation–oxygen bonds was estimated from the bond strength; the results indicate that the covalency of the Ga—O bond is greater than that of the Al—O bond. The peculiar cation distributions observed in the Y3Al5−xGaxO12 garnet solid solutions are most probably caused by the strong covalency of the Ga—O bond and also simultaneously induced by the need to decrease the cation–cation repulsive force. Crystal data: cubic, Ia3¯d, Z = 8, Mo Kα, λ = 0.71069 Å; at x = 0.0 (triyttrium pentaaluminium dodecaoxide): a0 = 12.0062 (5) Å, V = 1730.7 (2) Å3, Dx = 4.56 Mg m−3, Mr = 593.613, μ = 21.21 mm−1, F(000) = 2224, R = 0.029 for 294 reflections; at x = 1.0 (triyttrium tetraaluminium gallium dodecaoxide): a0 = 12.0432 (7) Å, V = 1746.7 (3) Å3, Dx = 4.84 Mg m−3, Mr = 636.351, μ = 24.09 mm−1, F(000) = 2368, R = 0.022 for 124 reflections; at x = 2.0 (triyttrium trialuminium digallium dodecaoxide): a0 = 12.0926 (9) Å, V = 1768.3 (4) Å3, Dx = 5.10 Mg m−3, Mr = 679.089, μ = 26.85 mm−1, F(000) = 2512, R = 0.018 for 144 reflections; at x = 3.0 (triyttrium dialuminium trigallium dodecaoxide): a0 = 12.1552 (6) Å, V = 1795.9 (3) Å3, Dx = 5.34 Mg m−3, Mr = 721.827, μ = 29.43 mm−1, F(000) = 2656, R = 0.018 for 184 reflections; at x = 4.0 (triyttrium aluminium tetragallium dodecaoxide): a0 = 12.2123 (8) Å, V = 1821.3 (4) Å3, Dx = 5.58 Mg m−3, Mr = 764.565, μ = 31.97 mm−1, F(000) = 2800, R = 0.014 for 159 reflections.