In earlier studies the behavior of single catalyst pellets of Pd on alumina has been investigated for the reaction of acetylene in an ethylene stream with hydrogen. Particle runaway, temperature over- and undershoots and chemically induced temperature oscillations have been observed. After that, the steady state and dynamic behavior of an adiabatic packed bed reactor has been studied experimentally. Temperature profiles of both the gas and solid phase as well as local temperature differences between the two phases were measured. Also here the temperature in the reaction zone exhibited oscillatory behavior. On addition of CO. the oscillations disappeared and the selectivity improved. For a given set of operating conditions. there existed a relatively small range of CO contents with good selectivity and satisfactory conversion. This range depends strongly on the inlet temperature, The dynamic response of the reactor to changes in the CO content showed a considerable wrong-way behavior. This high sensitivity to fluctuations in the CO content, found for our experimental reactor. indicates a probable cause for a thermal runaway in industrial practice. Recommendations for a stable reactor operation are given.