The cross contamination of Na form resins from the regeneration step in a mix-bed ion exchange column for pure water production processing will affect the quality of the effluent. For the prediction of the effect of the contamination on performance of a regenerated column, the breakthrough curve of the column packed with Na form, NH4 form and H form cationic resins is studied. The ion exchange equilibria in the H-Na-NH4 system is explained by a simple form derived from selectivity coefficients of K-H(Na) and K-H(NH4). The packing conditions of H form, Na form, and NH4 form cationic resins in a laboratory scale column were studied by stacking in layers and mixing in random order. The breakthrough curve depended on the packing conditions and was analyzed with a mathematical model in order to understand the local ion exchange behaviors. The effect of the packing conditions could be well simulated by using adequate initial conditions representing the packing conditions of resins in the column. It is shown that the homogeneous initial conditions of the mixing ratio of Na form resin is useful for the prediction of the effluent quality in an industrial scale column.