This work discusses the effect of the crystal structure and microstructure on the electrocatalytic activity of Lanthanum-Barium cobaltite (LBC), evaluatedas IT-SOFC cathode. Two systems with similar microstructures (particle sizes d_p≈1-5 µm) are used to compare the effect of the crystal structure: the cubic La_0.5Ba_0.5CoO_3-δ with La/Ba cations randomly distributed, and a tetragonal LaBaCo₂O_6-δ with La/Ba layered distribution. In addition, the effect of microstructure is studied by using a newsol-gel route which allows obtaining LaBaCo₂O_6-δ with smallest particle size (d_p≈500 nm). The electrode reaction isstudied by electrochemical impedance spectroscopy (EIS) as a function of temperature (T) and oxygen partial pressure (pO₂). The electrode polarization resistance (R_C,P) presents two contributions, a low frequency (R_LF) and a high frequency Gerischer-resistance (R_G). On one side, R_LF, associated to the O₂-gas diffusion across the gas layer boundary, presents low activation energy (E_a) and is proportional to pO₂^-1, presenting the same values regardless the microstructure. On the other side, R_G is in agreement with an O₂-reduction mechanismco-limited by O-surface exchange and O-bulk diffusion. R_G is strongly influenced by structure (trough the diffusion coefficient D_V) and microstructure (inversely proportional to d_p). Besides, R_G decreases by reducing d_p and by modifying the La/Ba ordering from layered to randomly distributed.