To systematically investigate the influence of electrolyte substrates on Sr-segregation and SrSO4 formation in (LaSr)(CoFe)O3 (LSCF) cathodes in solid oxide fuel cells, model thin films were grown on Gd-doped ceria (GDC) and on Y-doped BaZrO3 (BZY) electrolytes by pulsed laser deposition and heat treated at 800–1000 °C in synthetic air with a trace amount of SO2. A severe SrSO4 formation was observed in LSCF on GDC as compared with the BZY, especially at low temperature. The difference in Sr-segregation and SrSO4 formation on the LSCF was discussed in relation to Sr diffusion and related elemental redistribution across the interfaces.