Monoclonal antibody (MAb) variants differing by one or two C-terminal lysine residues can be separated by cation-exchange chromatography due to the difference in their charge distribution. The adsorption of the three MAb variants on a weak cation-exchange resin was characterized using directly the raw mixture in spite of the presence of some impurities. The effects of both, pH and eluent salt concentration, on the adsorption isotherm were investigated. Under certain experimental conditions distorted peak shapes and even peak doubling for single variant injections were obtained, in addition to unexpectedly long retention times. These observations were explained based on equilibrium theory. The separation of the MAb variants was designed for an isocratic and a linear salt gradient operation. The corresponding optimal values of pH and salt concentration were determined. The use of salt gradients not only allows reducing the process time and increasing enrichment of the variants, but also leads to some loss in purity. A baseline separation could be obtained under isocratic and strongly adsorbing conditions at pH 6.3. A lumped kinetic model and a procedure for estimating the corresponding parameters were developed and validated by comparison with experimental elution chromatograms in a wide range of operating conditions.