The axial dispersion coefficient of hexachlorobenzene in supercritical carbon dioxide was investigated in a fixed bed packed with glass beads. The on-line chromatographic pulse-response experiment was used in order to study the dynamics of a packed column under supercritical conditions. The radial dispersion was assumed negligible because of the packed column geometry. To estimate the axial dispersion coefficient, a pulse input of tracer/supercritical fluid (hexachlorobenzene/carbon dioxide) mixture was injected into the column and the effluent peak was analyzed using the moments of the chromatographic curve in the Laplace domain. The range of the operating conditions for temperature, pressure, and flow rate of supercritical fluid was 25-500degreesC, 1200-4000 psia, and 120-160 ml/h, respectively. The experimental data indicated that the axial dispersion coefficient is a function of temperature, pressure, and flow rate. The axial dispersion coefficient decreased with increasing temperature and increased with increasing pressure. This trend may be due to the increase of the density and viscosity of the supercritical carbon dioxide. Further-more, the axial dispersion coefficient increased with increasing interstitial velocity. These results may suggest that the contribution by convection is more important than molecular diffusion under supercritical operations. In order to investigate the authenticity of the dynamic model and also to investigate the extent of accuracy of the moment analysis which was used for parameter estimation, the experimental response peak was compared with the dimensionless theoretical (numerical solution) curve. The small deviation between the two curves is well within the experimental error of axial dispersion coefficient measurements. (C) 2004 Elsevier B.V. All rights reserved.