High resolution powder neutron diffraction has shown that the room temperature structure of the new fast oxide ion conducting solid electrolyte La0.9Sr0.1Ga0.8Mg0.2O2.85 is monoclinic, rather than orthorhombic, as for the undoped parent phase LaGaO3. In both systems the main distortion of the structure from the ideal perovskite is the tilting of the GaO6 octahedra, and the symmetry change has a direct effect on this tilting, such that for LaGaO3 the [001](p) tilt is in-phase when successive octahedra are considered, while for La0.9Sr0.1Ga0.8Mg0.2O2.85 it is antiphase. The degree of tilting of the octahedra is also reduced by the doping. For this room temperature data, the oxygen vacancies in La(0.9)Sr(0.1)Ga(0.8)Mg(0.2)O(2.85)are shown to be preferentially located in the apical sites.