A phase diagram in the parameter space of temperature and blend composition that contains order-disorder transition (ODT) and thermoreversible macrophase separation between constituent copolymers was constructed for binary blends of polystyrene-block-polyisoprene (PS-PI) copolymers. The constituent copolymers, designated as H102 and FS-1, have nearly symmetric copolymer compositions and rather different molecular weights; i.e., number-average molecular weight (M-n) and volume fraction of PS-block (f(PS)) are 1.0 x 10(5) and 0.47 for H102 while 2.1 x 10(4) and 0.40 for FS-1. The ODT was observed on the neat FS-1 and two blend specimens whose weight fractions of H102, Phi (H102), are 0.1 and 0.2. Counterintuitively, the ODT temperature (T-ODT) decreased with increasing the average molecular weight of the specimen, namely, T-ODT for FS-1 neat > T-ODT for the blend Of Phi (H102) = 0.1 > T-ODT for the blend of Phi (H102) = 0.2. Then T-ODT abruptly increased between the blends Of Phi (H102) = 0.2 and 0.3 and became inaccessibly high temperature (i.e., > 200 degreesC). The macrophase separation between H102 and FS-1 was observed on the blend Of Phi (H102) = 0.2. This macrophase separation was thermoreversible but accompanied by a hysteresis between cooling and heating processes. Although both H102 and FS-1 neat copolymers showed lamellar morphology, PS cylindrical morphology was observed on certain blend specimens. The origin of PS cylindrical morphology may be attributed to the slight asymmetry in the f(PS) of FS-1.