Among standard high temperature cathode materials for SOFC devices, La0.8Sr0.2MnO3- (LSM) displays the least reactivity with the oxide-ion conductor La2Mo2O9 (LMO), yet reaction is observed at high processing temperatures, identified by X-ray diffraction (XRD) and FIB-SIMS studies after annealing at 1050°C and 1150°C. Additionally Sr and Mn solutions were deposited and annealed on LMO pellets, as well as a Mo solution on a LSM pellet. From these studies several reaction products were identified by XRD and located by FIB-SIMS on the surface of pelletized samples and by depth profiling it was evident that reactivity extended up to ~10 m from the surface region. When strontium was present, a SrMoO4-type scheelite phase was always observed as a reaction product, and when manganese was present, LaMnO3+ single crystals were observed on the surface of the LMO pellets. Depending on configuration and annealing temperature, additional phases such as La2MoO6 and La6MoO12 have also been detected. Reaction mechanisms and detailed reaction formulae are proposed to explain these observations. The strongest driving force for cationic diffusion appears to originate from Mo6+ and Mn3+ cations, rather than from Sr2+.