The structures of γ- and β-Li_2.5Ga_0.5GeO₄, have been determined using Rietveld refinement of X-ray powder diffraction data. They are stoichiometric derivatives of γ- and β-Li₃PO₄. The arrangements of GeO₄/PO₄ tetrahedra are the same in each case, but the Li sites, especially Li(2), contain a disordered mixture of Li and Ga in γ- and β-Li_2.5Ga_0.5GeO₄. Such cation disorder is unusual in a stoichiometric phase. The γ phase may be regarded as entropy-stabilised since it exists under equilibrium conditions only at high temperatures, ca. 1200 °C, just below melting: the size mismatch between Li⁺ and Ga³⁺ ions and the likely unfavourable enthalpy of mixing associated with Li/Ga disorder is offset by the large entropy contribution to the Gibbs energy at high temperatures. A discussion of possible mechanisms for the β⇌γ transition is given.