A unique phase behavior was investigated for binary mixtures of diblock copolymers in order to explore the cosurfactant effect as a function of temperature. The mixtures are composed of a long, asymmetric polystyrene-block-polyisoprene (SI) copolymer and a short symmetric SI copolymer denoted, respectively, as as and s(3). The neat as copolymer forms polystyrene (PS) spherical microdomains on a body-centered-cubic lattice in polyisoprene (PI) matrix from ambient temperature to 180 degrees C, while neat s(3) is always in the disordered state in the same temperature range. The mixtures of as/s(3) = 45/55 and 75/25 (w/w) are mainly focused in this study. Both of them show lamellar morphology at low temperatures, then undergo an "order-order transition (OOT)" to a "bicontinuous structure" with a considerable distortion in the long-range order, and finally reach the truly disordered state through the "order-disorder transition (ODT)" with increasing temperature. The reason why quotation marks were put in "OOT" and "ODT" is explained in the text. We found that the invariants (the integrated scattered intensity) do not change across the "OOT" for these two mixtures. It is intriguing to note that these two mixtures exhibit a quite opposite temperature dependence of the characteristic length D for the ordered structures, which seems to be related to the "cosurfactant effect" as discussed in the text. The cosurfactant effect was shown to decrease with increasing temperature and to promote the distorted bicontinuous phase.