A series of samples in the (\mathrm{Gd}{1-x} \mathrm{Sr}{x} \mathrm{CoO}{3-y}(x=0.00,0.25,0.50,0.75), and 1.00()) system with perovskitetype oxide have been prepared at (1200^{\circ} \mathrm{C}) under an atmosphere with and without oxygen gas flow. X-ray diffraction analysis of the solid solution assigns the structure of the compositions of (\boldsymbol{x} \mathbf{0 . 0 0}), 0.25 , and 1.00 to the orthorhombic system, (x=0.50) to the cubic system, and (x=0.75) to the tetragonal system. The reduced lattice volume of the system is increased as the (x) value gets higher. The amount of (\mathrm{Co}^{4+}) ion or the mixed valence state between (\mathrm{Co}^{3+}) and (\mathrm{Co}^{4+}) ions is quantitatively analyzed by the iodometric titration method. The mole ratio of the (\mathrm{Co}^{4+}) ion, the (\tau) value, is maximized at the composition of (x=0.50) and the (y) value, the oxygen deficiency, increases as the (x) value increases. Nonstoichiometric chemical formulas are obtained with the (x, \tau), and (y) values. The electrical conductivity of the (\mathrm{GdCoO}{1.00}) sharply increases within the experimentaf temperature range; therefore, the activation energy of this compound is higher than those of the other compositions. The electrical conductivity measurement of the composition ((x=0.50)) indicates a metallic property. The compounds of the compositions ((x=) (0.00 \sim 0.75) ) show a paramagnelic properly. The magnetic measurement shows that the paramagnetic Curie temperature (\left(\theta_{p}\right)) is a positive value in the case of (x=0.50) with the maximum amount of the (\mathrm{Co}^{41}) ion. The (\mu_{\mathrm{cff}-\mathrm{C}}) is decreased as the (\tau) value gets higher. The (\mathrm{SrCOO}_{2.52}) has the Neel temperature of 550 K. O 1993 Academic Press, fnc.