The effective diffusivity of O-2-N-2 in porous media was measured at high temperatures (650-800degreesC) using an electrochemical concentration cell. Porous membranes having total porosity between 29 and 48 vol% were fabricated from Sr-doped LaMnO3 (LSM) with 20 to 30 wt% carbon added as a pore former. The O-2-N-2 effective binary diffusivity, D-O2-N2(eff), at 800degreesC increased from similar to0.016 to similar to0.12 cm(2)/s with increasing open porosity between 15 and 44 vol%. The D-O2-N2(eff) exhibited a nonlinear dependence on open porosity and increased dramatically for samples with greater than 35 vol% open porosity. The estimated effective Knudsen diffusivities of O-2 and N-2, D-O2K(eff) and D-N2K(eff), at 800degreesC were an order of magnitude higher than the effective binary diffusivity, D-O2-N2(eff). Thus, O-2-N-2 transport through the porous membranes was governed by the effective binary diffusivity, D-O2-N2(eff). The effects of O-2-N-2 effective binary diffusivity, D-O2-N2(eff), on concentration polarization of cathodes for solid oxide fuel cells were assessed. The nonlinear behavior of the O-2-N-2 effective diffusivity as a function of open porosity indicates that a critical amount of porosity in the cathode is necessary to ensure that the overpotential due to concentration polarization is small. The temperature dependence of D-O2-N2(eff) was investigated between 650 and 800degreesC, which was found to be in accord with the Chapman-Enskog model. (C) 2003 The Electrochemical Society.