Internal partial pressure in BCY based solid oxide fuel cells were measured as a function of electrolyte thickness. The measurements of internal  ΔΦ were conducted on the thin (~ 10 µm) and thick (~ 50 µm) BCY electrolyte using an embedded Pt probe, and it was observed that  ΔΦ depends on the electrolyte thickness. Using the values of internal ΔΦ, internal hydrogen and oxygen partial pressure were calculated, and it was found that the internal partial pressure changes with electrolyte thickness. Thin BCY cells are dominated by the near-cathode gases rather than the near-anode gases, indicating it is more vulnerable to water vapor produced at the cathode during fuel-cell operation. The correlation between internal partial pressure profile and performance durability were also investigated. CC tests were conducted on a thin (~ 10 µm) BCY-electrolyte cell with BCY + LSCF cathode, and on a thick (~ 35 µm) BCY electrolyte cell with BZY + LSCF cathode, at positive and negative cell voltage. The former showed a rapid rate of voltage drop under all operating conditions, while the latter showed much more stable performance, even when operating at a negative voltage. No changes in microstructure and composition were detected in the thick BCY electrolyte or the BZY + LSCF cathode. Thus, the present work shows the effect of the electrolyte thickness and cathode-material selection on the durability of BCY based cells, in terms of internal partial pressure.