The crystal structure and properties of La 1 - x Sr x CoO 3 - y with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2-400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO 3 is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co 4 + (cobalt ions with one positive effective charge, Co C o ) and oxygen vacancies (V O ). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers - and the resultant electrical conductivity - decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations - probably between oxygen vacancies and strontium ions - and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.