Axial dispersion in upward gas flow is investigated by pulse and displacement experiments in a vertical, packed column with different concentrations of the tracer and at pressures up to 1.5 MPa. The responses to the introduced pulse and step changes are measured at two locations and the extent of axial dispersion, represented by the Bodenstein number, is determined by curve fitting in the time domain. The performed experiments demonstrate that the residence time distribution is considerably affected by density differences between the tracer and carrier gas, particularly at elevated pressures. Obtained Bodenstein numbers for step changes from nitrogen to a helium/nitrogen mixture and vice versa differ by as much as a factor ten, depending on the helium concentration and column pressure. The difference in axial dispersion may be ascribed to gravitation-driven instabilities as due to vertical density gradients in the case of a heavy gas displaced by a light gas : density gradients in the step changes from a light to heavy gas evidently inhibit axial dispersion. The presented observations are of major importance for the description of flow behaviour of gases in packed bed reactors where density gradients exist due to temperature and concentration gradients, particularly because many processes operate at elevated pressures.