First-principles and statistical-theory calculations were applied to examine the interactions between oxygen molecules and the (100) surfaces of LaMnO 3 and La 0.5Sr 0.5MnO 2.75, one of the most-used cathode materials in solid oxide fuel cells (SOFCs). To predict the rate constants for the interactions between O 2 and LaMnO 3 or La 0.5Sr 0.5MnO 2.75, potential energy profiles were constructed using the nudged elastic band (NEB) method. Predicted rate constants for the dissociation of adsorbed oxygen species on LaMnO 3 (lm) and La 0.5Sr 0.5MnO 2.75 (lsm) can be expressed as k diss,Im) 2.35 × 10 12 exp(-0.50 eV/RT) s -1 and k diss,lsm) 2.15 × 10 12 exp(-0.23 eV/RT) s -1, respectively, in the temperature range of 873-1273 K at 1 atm. Because the activation energy for oxygen dissociation on La 0.5Sr 0.5MnO 2.75 (0.23 eV) is much smaller than that on LaMnO 3 (0.50 eV), oxygen vacancies greatly enhance O 2 dissociation kinetics. The kinetic and mechanistic studies for the interactions at the molecular level are imperative to gaining a fundamental understanding of oxygen reduction kinetics on cathode materials and to providing important insight into the rational design of more catalytically active cathode materials for SOFCs.