X-ray computed tomography was used to visualize the interior of a catalyst ink composed of platinum/carbon black in water/1-propanol over time. Bubble expansion and the emergence of a tertiary phase were observed. Gas chromatography-mass spectrometry determined that propionaldehyde and propionic acid were produced in the ink. Analysis of the gases within the ink container indicated that the CO2 concentration was increased. These results indicate that the oxidation reaction of 1-propanol, as catalyzed by platinum, resulted in degradation of the ink. Thus, controlling the exposure of the catalyst ink to oxygen is an important aspect of managing the quality of the ink. (C) 2018 The Electrochemical Society.