Seven compositions of (La1-xSrx)0.99FeO3-δ (x = 0, 0.05, 0.15, 0.25, 0.35, 0.50, 0.70) perovskites were synthesized using the glycine-nitrate method. The (La1-xSrx)0.99FeO3-δ compounds were characterized with powder X-ray diffraction, dilatometry, total conductivity and electrochemical impedance spectroscopy on cone-shaped electrodes using a Ce1.9Gd0.1O1.95 electrolyte. Dilatometry shows that the thermal expansion coefficient of the (La1-xSrx)0.99FeO3-δ compounds increases with increasing strontium content. Total conductivity finds is maximum for LSF35. The activity of the (La1-xSrx)0.99FeO3-δ based perovskites towards the electrochemical reduction of oxygen was strongly dependent of the strontium content, the activity being highest for the composition (La0.85Sr0.15)0.99FeO3-δ. The results indicates that Fe(III) is the catalytic active specie towards the electrochemical reduction of oxygen in a solid oxide fuel cell on (La1-xSrx)0.99FeO3-δ compounds. The results also show that oxide ion vacancies in the perovskite structure are important for the electrochemical reduction of oxygen. However, a negative effect of defect ordering (Sr2+ and oxide ion vacancies) is strongly indicated for the strontium rich compounds. Segregation of SrO to the surface might also play a detrimental effect on the activity towards the oxygen reduction reaction.