The contrast between the saturation tetragonal magnetostriction, λ^γ,2 = (3/2)λ₁₀₀, of Fe_1−xGa_x and Fe_1−yGe_y, at compositions where both alloys exhibit D0₃ cubic symmetry (second peak region), was investigated. This region corresponds to x = 28 at. % Ga and y = 18 at. % Ge or, in terms of e/a = 2 x + 3 y + 1, to an e/a value of ∼1.55 for each of the alloys. Single crystal, slow-cooled, ternary Fe_1−x−y Ga_xGe_y alloys with e/a ∼1.55 and gradually increasing y/x were investigated experimentally (magnetostriction, elasticity, powder XRD) and theoretically (density functional calculations). It was found that a small amount of Ge (y = 1.3) replacing Ga in the Fe–Ga alloy has a profound effect on the measured λ^γ,2. As y increases, the drop in λ^γ,2 is considerable, reaching negative values at y/x = 0.47. The two shear elastic constants c′ = (c₁₁− c₁₂)/2 and c₄₄ measured for four compositions with 0.06 ≤ y/x ≤ 0.45 at 7 K range from 16 to 21 GPa and from 133 to 138 GPa, respectively. Large temperature dependence was observed for c′ but not for c₄₄, a trend seen in other high-solute Fe alloys. The XRD analysis shows that the metastable D0₃ structure, observed previously in slow-cooled Fe–Ga at e/a = 1.55, is replaced with two phases, fcc L1₂ and hexagonal D0₁₉, at just 1.6 at. % Ge. The two are the stablephases of the assessed Fe–Ga phase diagram at x ∼ 28. Notably, at y = 7.8, only the D0₃phase (the equilibrium phase of Fe–Ge at e/a = 1.54) was found in the ternary alloy. The theory also shows that the D0₃ instability is removed for compositions with y ≥ 3.9, when D0₃ becomes the structure’s ground-state phase. Thus, the high, positive λ^γ,2 value for Fe–Ga at x = 28 could be the result of the high sensitivity of its metastable D0₃ structure.