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Summary The velocity dependence of parameters in the Coats-Smith model for tracer dispersion and tailing in porous media was investigated. An axisymmetric pore with a step change in cross-sectional area was used as a model system to determine whether the stagnant zone resulting from variations in flow cross sections contributes to the observed capacitance (i.e., tailing) at the pore level. Numerical simulations show that eddies with recirculation flow are formed in the pockets as a result of flow separation. The tracer transport between the eddies in the dead zones and the main channel was found to be diffusion-limited. The simulations reveal that in the Stokes flow regime, the mass-transfer coefficient between the two regions is independent of the interstitial velocity, in contradiction to earlier theories. Coreflood experiments were performed with radioactive tracers to verify the hypothesis that the capacitance effects are not the result of a change in flowing fraction. The experimental results confirm that tracer tailing is a function of the ratio of the molecular diffusivity to the flow rate. in light of these findings, we investigated the validity of the Coats-Smith model to predict dispersion and tailing in a porous medium. Our studies show that the Coats-Smith model may be used; however, certain restrictions, described in this paper, apply to the procedure for estimation of parameters.