The reaction kinetics and mechanisms between 8 mol % yttriastabilized zirconia (YSZ) and 30 mol% Sr-doped lanthanum manganite (La0.65Sr0.30MnO3, LSM) with A-site deficiency for the application of planar solid oxide fuel cells (SOFCs) were investigated. The LSM/YSZ green tapes were cofired from 1200 to 1400 degreesC for 1 to 48 h and then annealed at 1000 degreesC for up to 1000 h. The results showed that the diffusion of manganese cations first caused the amorphization of YSZ, and then the formation of small La2Zr2O7 (LZ) or SrZrO3 (SZ) crystals if treated for a longer time at 1400 degreesC. The ambipolar diffusion of the Mn-O pair, transported through the migration of oxygen vacancy, plays an important role in the formation of secondary phases. The diffusion of LSM to YSZ and substitution of Mn for Zr both result in the enhanced concentration of oxygen vacancy, leading to the formation of a void-free zone (VFZ). No additional reaction products in annealed LSM/YSZ specimens, treated at 1000 degreesC for 1000 h, were detected. The interfacial reactions, detailed reaction kinetics, and mechanisms are reported.