The resistivity of the as-fabricated thermistor material, nickel-iron-manganite, changes during initial aging in the temperature range of 150-300ºC before becoming stable.X-ray photoelectron spectroscopy (XPS) was used to determine if any valency change or chemical shift of the cations or oxygen occurred during aging. The goal of the study was to identify any ionic changes that might affect thermistor stability. The only observed changes in 2p
3/2 peaks due to aging were those related to Ni ions; the same peaks for Mn, Fe, and the 0-Is peak were unchanged. The changes in the Ni 2p
3/2 peak may possibly be related to: (a) the migration of Ni2+ ions from octahedral to tetrahedral sites, (b) subtle changes in the energy states of Ni2+ which promoted a more stable ionic structure, or (c) the presence of Ni3+ ions, some of which revert to Ni2s+.