Localized hot spots have been observed in industrial reactors during the catalytic hydrogenation of acetylene present in ethylene-rich mixtures used as feed to polymerization reactors. It is important to determine the cause of these hot spots. Infrared imaging revealed that the transversal temperature on top of a shallow packed bed was not uniform during the hydrogenation of mixtures of ethylene and acetylene. Temperature patterns were observed only under relatively high conversion of ethylene, which is rather different from the operation of commercial acetylene purification reactors. Two qualitatively different nonuniform temperature patterns were observed: (a) antiphase oscillatory patterns and (b) nonuniform stationary hot zones. The periodic antiphase behavior, observed in approximately 60% of the experiments, consisted of a long period (35-50 min) during which the catalyst temperature was essentially at a pseudostationary state. Then, during a rather short period (3-5 min), the hot region moved from one side of the catalyst bed to another and then returned to the original location. The amplitude of the temperature oscillations ranged from 3 to 30 degrees C. They were observed for maximum catalyst temperatures between 78 and 161 degrees C and ethylene feed concentrations greater than or equal to 22.25 mol %. The oscillations occurred in both the presence and absence of carbon monoxide in the feed. When antiphase oscillatory behavior did not occur, a nonuniform stationary state was observed in which one or two hot zones existed on the surface of the catalyst bed. The location of the hot zones was influenced by nonuniformities of the catalyst in the bed and occurred for conditions in which a unique steady state existed. The observation of moving hot zones in the shallow packed bed seems to confirm recent theoretical predictions that hot zones are likely to form when the catalytic reaction rate may oscillate under constant ambient conditions.