Hydrodynamic dispersion significantly impacts solute transport in a porous medium, such as a packed bed reactor or a soil column. Recent developments in the area of supercritical fluid extraction and chromatography have created a need for understanding dispersion phenomena in such systems. Because the literature in this area is sparse and limited to convection-dominated conditions, a supercritical fluid system was constructed for measuring dispersion coefficients in porous media at low Reynolds numbers (Re (0.1). The apparatus allowed quantification of the axial dispersion of methane in supercritical carbon dioxide by imperfect pulse chromatography. Flow rate (interstitial velocity) was varied for a variety of solid types. A range of size fractions including Borden sand (nominal diameter, d N "0.01, 0.015, 0.033 cm and bulk sample), Moffet aquifer sand (d N "0.033 cm) and spherical glass beads (d N "0.015 cm) were employed. With the temperature fixed at 45ยฐC, pressure was varied from 140 to 450 atm. Experimental Re values ranged from 0.01 to 3. As expected, mechanical dispersion dominated over diffusion at higher flow rates (approximately Re'0.4, diffusion-based Peclet number, Pe B '3), and measured dispersion coefficients agree well with those reported elsewhere. The dispersion coefficients measured for the lower flow rates (approximately Re(0.1, Pe B (0.4) demonstrate diffusion dominance over mechanical dispersion. The observed dispersion coefficients are demonstrated to be greater than those typical of liquid systems and less than those for gaseous systems for particular Re or Pe B values. However, the dispersion behavior is consistent with that reported for gases and liquids when the observed dispersion coefficients are scaled by the appropriate diffusion coefficients. Experimentally determined dispersion coefficients are correlated with Pe B ("Re Sc) for values ranging from 0.02 to 30. The fitted expression captures the dynamics of the transition from diffusion to convection-dominated dispersion under supercritical conditions and is consistent with previously reported expressions for liquids and gases.