Nucleation and growth of lanthanum zirconate (LZO) between (001) single crystals of 9.5 mol% Y2O3-stabilized ZrO2 (YSZ) and porous La0.85Sr0.15MnyO3=delta (y = 0.95... .10) perovskites were investigated. The perovskites were screen-printed on the solid electrolyte and sintered in air at 1373 K for short periods. High resolution transmission electron microscopy, electrochemical impedance spectroscopy, and atomic force microscopy were employed for interface characterization. The manganese concentration in the perovskite affects the onset of nucleation and the growth rate of lanthanum zirconate. Excess lanthanum oxide within the perovskite reacts immediately with YSZ to form dense LZO layers. Layer growth kinetics is controlled by bulk diffusion of cations. Stoichiometric perovskite leads to the reductive formation of cubic LZO islands at the cathode/electrolyte interface after a few minutes of sintering, The A-site deficient perovskite is characterized by the reductive dissolution of Mn into YSZ in the early sintering stage and by a pronounced surface diffusion of cations. After the critical Mn concentration in the perovskite is reached, reductive nucleation of LZO originates. Island growth is controlled by surface diffusion of cations and the supply of lanthanum. The electrochemical properties of the cathode/YSZ interfaces are strongly influenced by the formation of LZO at the triple phase boundary. Charge transfer, dissociation of adsorbed molecular oxygen ions, and surface diffusion of atomic oxygen are discriminated from electrochemical impedance data.