A model system of short polystyrene-polyisoprene diblock copolymers mixed with the corresponding strongly segregated long diblock copolymers is studied by small-angle X-ray scattering, electron microscopy, and self-consistent mean-field calculations of the ordered microphases. It is found that most of the small diblocks solubilize in the ordered domains, but a small amount segregates to the domain interfaces, in the manner of cosurfactants. Adding more short diblocks results in domain size shrinkage, consistent with a theoretical analysis of the changes in the interfacial energy and the chain stretching. The stability of the lamellar phase is discussed with respect to the structural fixed point in sections of the phase cube for diblock mixtures studied earlier. More complex systems, involving nonionic surfactants in solutions of water-soluble block copolymers, are expected to behave in the same way.