In compound-bed, due to the evenly blending and closely contacting of anion resin and  cation resin, nearby the cation resion is the anion resin, therefore, each pair of anion and cation resion  is equivalent to stage-one compound- bed, in this way the compound bed can be regarded as  the combination of countless stage-one compound- beds, due to closely contacting and evenly blending of anion and cation resin, when  water passes through the exchanger, the anion and  cation ions in water exchanges the production reaction with the anion and cation resin at the same time:

Within the exchanger, there are countless pairs of anion and cation resin, during operation, the cation resion absorbs the cation ions to release OH, and H and OH react to generate H2O.