Geometrical considerations about macromolecular size have been used in order to make a critical analysis of the minimum amount of block copolymer needed to saturate the interface in the melt mixing of immiscible polymer blends. For the case of dispersed spherical domains an expression has been developed (Equation (8)) which contains only molecular parameters such as the number of bonds, the characteristic ratio and the composition of the block copolymer used to compatibilize the blend. The shell surrounding each particle was divided into small pseudocubical elements. the unperturbed size of the matrix-like blocks of the copolymer was considered as representative of the size of these elements. Comparison has been made with experimental data regarding an ethylene-propylene rubber dispersed in a polystyrene matrix. That blend was compatibilized with styrene-hydrogenated butadiene copolymers with different molecular weights and architecture (diblock and triblock). The experimental data allow a quantitative assessment of the number of compatibilizer chains per unit cubical element corresponding to the onset of interfacial saturation : this value was found to depend on the copolymer architecture but is substantially independent of the copolymer molecular weight.