Strontium migration from the oxygen electrode to the interface between the electrolyte and interlayer was investigated in solid oxide cells in fuel cell (SOFC) and electrolysis (SOEC) modes. Four samples were imaged by focused ion beam scanning electron microscopy (FIB-SEM) serial sectioning. After reconstruction, the spatial distribution of the strontium zirconate secondary phase was examined by measuring the volume fractions, phase size distributions, interfacial surface areas and the shape of each detected distinct inclusion. The analysis shows that the accumulation lingers during operation. The results also suggest that the inclusions tend to propagate towards the GDC/YSZ interface. The detrimental effect on the performance was assessed using 3-D finite element transport analysis, by comparing the effective conductivity computed with and without the presence of the secondary phase. The operation in SOFC mode caused a limited decrease of the conductivity, while the effects on the performance are more significant for SOEC operation.