The steady state and dynamic behaviour of an adiabatic packed bed reactor for the selective hydrogenation of ethyne and ethene mixtures was studied experimentally. Different methods to achieve adiabatic conditions on a laboratory scale were tested. Temperature profiles of both the gas and solid phase as well as local temperature differences between the two phases were measured. For hydrogenation in the absence of carbon monoxide the selectivity was always poor. Also, in that case, the temperature in the reaction zone exhibited oscillatory behaviour. 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 behaviour. 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.