The stability and NOx sensing performance of electrochemical cells of the structure Sr-doped LaCrO3-delta (LSC)/yttria-stabilized zirconia (YSZ)/Pt are being investigated for use in NOx aftertreatment systems in diesel vehicles. Among the requirements for NOx sensor materials in these systems are stability and long lifetime (up to 10 years) in the exhaust environment. In this study, cell aging effects were explored following extended exposure to a test environment of 10% O-2 at operating temperatures of 600-700 degrees C. The data show that aging results in changes in particle morphology, chemical composition, and interfacial structure. Impedance spectroscopy indicates an initial increase in the cell resistance during the early stages of aging, which is correlated principally to densification of the Pt electrode. Also, X-ray photoelectron spectroscopy indicates formation of SrZrO2 solid-state reaction product in the LSC, a process which is of finite duration. Subsequently, the overall cell resistance decreases with aging time, due in part to roughening of YSZ-LSC interface, which improves interface adherence and enhances charge transfer kinetics at the gas phase/YSZ/LSC triple-phase boundary. This study constitutes a first step in the development of a basic understanding of aging phenomena in solid-state electrochemical systems with applications not only to sensors, but also to fuel cells, membranes, and electrolyzers.