Mixed oxide-ion and electron conductors (MIECs) are a scientifically and technologically important class of functional materials for energy conversion and storage. The present work reports a study on transport properties of two well-known MIECs, SrCo0.9Nb0.1O3-delta (SCN) and SrCo0.9Ta0.1O3-delta(SCT), using a combined experimental oxygen permeation measurement and theoretical modeling approach. The results suggest that SCN has a higher oxygen diffusivity, surface oxygen exchange rate constant and oxide-ion conductivity than SCT even though both materials have the same crystal structure. A molecular orbital energy analysis reveals that the slight difference in outer electronic shell configuration between Nb5+ and Ta5+ is responsible for the variance in transport properties.