The coexistence of two crystal modifications of lanthanum gallate, orthorhombic and rhombohedral, was revealed for the first time in two series of solutions: LaNi_xMg_0.2xGa_1–1.2xO_3−δ ([Ni]: [Mg] = 5: 1) and LaNi_xMg_0.5xGa_1–15xO_3−δ ([Ni]: [Mg] = 2: 1), x = 0.01–0.10. The unit cell parameters in the both series of solid solutions were refined by the Rietveld method. The magnetic susceptibility measurements point to the presence of clusters of Ni(III) atoms with strong antiferromagnetic interactions. The conductivity measurements point to high conductivity values for the system with the ratio [Ni]: [Mg] = 5 : 1.