Interfacial adsorption from a binary mixture of short and long polystyrene -polyisoprene diblock copolymers (PS-PI or its deuterated analogue dPS-PI) to interfaces of a PS(M = 3 x 10(6)) homopolymer was studied using nuclear reaction analysis. Short symmetric diblocks dPS(M = 10(4))-PI(M = 10(4)) (designated dS), or a similar protonated analogue (hS), and a highly asymmetric, long diblock dPS(M = 10(6))-PI(M = 10(4)) (L) were used in the binary mixtures. We found that the shorter diblocks adsorb preferentially at the interfaces : the isotopic contrast between dS and hS in the mixtures with L enables us to extract detailed information on the surface segregation of the short and long species. Our data are analyzed in terms of a modified Flory-type mean field model due to de Gennes and Leibler. Interfacial segregation from the binary diblock mixture is well predicted by this model with no adjustable parameters, using the adsorption characteristics determined earlier for systems with a single diblock component.