While porous mixed ionic-electronic conductors (MIECs) have been widely used as electrodes for solid-state ionic devices, the electrochemical processes occurring in such an electrode are still not well understood. Equivalent circuit analyses indicate that there are three parallel paths for electrode reactions to occur in a cell with porous MIEC electrodes : (i) the triple-phase boundary (TPB) between the electrolyte, the MIEC, and O-2 gas, (ii) the TPB between the current collector, the MIEC, and O-2 gas, and (iii) the interface between the MIEC and O-2 gas. The relative significance of each path is determined not only by the transport and catalytic properties of the MIEC but also by the interfaces between the MIEC and other cell components. The effectiveness of the MIEC/gas interface is influenced by the rates of several sequential processes : transfer of ionic defects across the MIEC/electrolyte interface, transport of ionic and electronic defects in the MIEC, reaction at the MIEC/gas interface, and transport of O-2 in the pores of the MIEC. Accordingly, oxygen diffusion in the MIEC and reaction at the MIEC/gas interface are only part, not the entirety, of the electrode kinetics. In general, the TPBs cannot be ignored without careful analysis or adequate justification. Further, as a first-order approximation, it is demonstrated that the thickness of the electrochemically active layer of a porous MIEC electrode, delta, varies dramatically with the properties of the MIEC. Typically, delta decreases with increased rate of surface reaction and with decreased transport of ionic or electronic defects in the MIEC. In the absence of ionic or electronic transport in the MIEC, for instance, delta approaches zero, i.e., oxygen reduction or evolution can occur only at one of the TPBs.