The influences of the oxygen partial pressure and the LSM/YSZ ratio on the LSM–YSZ interface reactions at 1,000 °C were investigated. Both pellets and diffusion couples were employed in the study. The equilibrium thermodynamics of the LSM–YSZ reactions was clarified based on the pellet study–powder reaction. LSM reacts differently with YSZ in different atmospheres. In air, m-ZrO2 (monoclinic) is formed; while in N2, SrZrO3 and/or La2Zr2O7 are formed depending on the initial LSM/YSZ ratio. The reactions are reversible with varying P(O2) i.e. treating the sample in air after the heat treatment in N2 results in a decomposition of the formed La- and Sr-zirconates. The de-stabilisation of the LSM–YSZ interface under long-term annealing at 1,000 °C originates mainly from the inter-diffusion across the interface. Under reduced P(O2), the Mn diffusion from LSM into YSZ is enhanced. High P(O2) (≥0.21 atm) promotes the m-ZrO2 formation.