The quantitative basis of the theory of buffer properties for two-phase acid-base buffer systems was derived. The analytical equations for buffer action with respect to all components β_i of acid-base pairs for diverse multicomponent systems were deduced. A remarkable relation of proportionality was found between quantities (Formula presented.). In heterogeneous systems the maximum buffer capacity (Formula presented.) occurs when (Formula presented.) is equal to (Formula presented.) , (Formula presented.) and (Formula presented.) are the dissociation and repartition constants, respectively). The nature of synergic buffer action in two-phase systems by formation of mixed species in organic phase was explained. Agreement between theoretical and experimental data is excellent for all the studied buffer systems.