The effect of velocity changes in fixed-bed adsorption of ternary mixtures (two absorbable components and one inert) is analyzed. In a diluted system governed by linear isotherms transition types are contact discontinuities. It is shown that the velocity variations (nontrace systems) resulting from changes in gas composition influence the transition types producing simple waves or shocks. Roll-up phenomena can appear in the adsorption step. Two models of fixed-bed adsorption are analyzed : a dispersed plug flour model and a plug flow model both with thermodynamic equilibrium between bulk gas phase and solid phase. The mathematical approach for the solution of the equilibrium model is based on the theory of characteristics. The axial dispersion model is solved by orthogonal collocation. Examples of fixed-bed adsorption in a clean bed and desorption of a saturated bed are shown.