For the first time by X-ray method two phases of the solid solutions LaCoxGa1-1.2xMg0.2xO3-δ and LaCoxGa1-1.5xMg0.5xO3-δ (x = 0.01 − 0.10) with different structure were found – rhombohedral and orthorhombic phases. On the basis of the data on evaporation of the components a synthetic procedure was advanced allowing the losses of cobalt to be minimized. The study of magnetic characteristics of obtained solid solutions showed the formation of high nuclearity clusters containing cobalt atoms, and also magnesium and associated vacancies even in diluted solid solutions. Clusters are characterized by a competition between ferro- and antiferromagnetic exchange interactions, whereas the long order exchange is antiferromagnetic.