Peclet numbers based on particle diameter (Bodenstein numbers) were determined for eight tracer/carrier gas pairs (selected from H-2, He, N-2 and Ar) flowing through a single-pellet-string column randomly packed with equilateral solid cylinders (tube diameter/particle diameter ratio=1.5). Fitting in time domain was used for extracting the axial dispersion coefficient, E(TC) the column responses to square-wave input signals. To obtain good match between theory and experiment, small ideal mixers were assumed to exist within the column in addition to axially dispersed plug flow and time delay. Contrary to the literature it appeard that Bo cannot be correlated solely with the product ReSc. The influence of the kind of tracer and carrier gases was empirically expressed either by the Schmidt number of the tracer-carrier (T-->C) pair or by the ratio of molecular weights of tracer and carrier gases. Using this correlation together with an extended form of the Edwards and Richardson equation, dispersion data for all T-->C pairs were successfully correlated.