This paper reports on the investigations of the chemical compatibility between SOFC cathode materials with compositions Pr0.8Sr0.2Co0.2Mn0.8O3-delta, Pr0.8Sr0.2Co0.2Fe0.8O3-delta, Pr0.8Sr0.2Co0.3Mn0.7O3-delta and Pr0.75Sr0.2Co0.2Mn0.8O3-delta and the electrolyte materials with compositions La0.8Sr0.2Ga0.9Mg0.1O3-delta, and La0.9Sr0.1Ga0.8Mg0.2O3-delta. The lanthanum gallate electrolyte with 20 mol.% Sr contained two additional phases, namely, LaSrGa3O7 and LaSrGaO4, while that with 10 mol.% Sr was formed in nearly single phase. Two types of experiments were performed: (a) reactivity experiments of powder mixtures and (b) diffusion experiments in cathode/electrolyte double-layer pellets. No reaction products were detected by XRD. High Co diffusion into the electrolyte was identified with SEM/EDX in all diffusion experiments examined. The transition metals diffuse in the order Mn < Fe < Co. La, Pr and Ga show a smaller tendency for diffusion. The diffusion of transition metal cations into the electrolyte La0.8Sr0.2Ga0.9Mg0.1O3-delta caused the destabilisation and disappearance of the second phases in the interdiffusion zone. In the case of the A-site deficient cathode, the formation of LaSrGa3O7 second phase was identified on the electrolyte side, near the interdiffusion zone.