Carbon deposition from CO and other carbon-containing fuels is a major cause of the performance degradation of catalysts and electrocatalysts in many energy conversion devices, including low-temperature solid oxide cells (LT-SOCs). In this work, we present direct observation of carbon deposition on thin-film CeO2 electrodes at LT-SOC operating temperatures (450 °C) in a CO/CO2 atmosphere by in operando X-ray photoelectron spectroscopy. In contrast to the general view that CeO2 is a carbon tolerant material, significant carbon formation was observed on CeO2 during CO2 electrolysis, with no other catalyst present. Moreover, carbon deposition on CeO2 demonstrated an intriguing threshold onset formation against surface Ce3+ concentration. With the aid of Monte Carlo simulations, we propose the neighboring Ce3+-Ce3+ pairs to be a critical catalytic structure that facilitates carbon deposition from CO. Finally, we propose mitigation of carbon deposition on CeO2 by doping CeO2 with non-redox-active cations, and proved this concept using 50% Gd- and 50% Zr-doped CeO2 as an example system. These findings provide an in-depth understanding of the mechanism of carbon deposition on CeO2 during electrochemical reactions and can guide the design of carbon-resistant CeO2-based electrocatalysts.