Temperature-composition phase diagrams for binary blends of chemically dissimilar diblock copolymers, (A-block-B) and (A-bloch-C) copolymers, were constructed experimentally. For the study, two polystyrene-block-polyisoprene (SI diblock) copolymers (SI-7/8 and SI-10/53) and three polystyrene-block-polybutadiene (SB diblock) copolymers (SB-9/8, SB-5/36, and SE-10/10) were used. Three binary blend systems were prepared: (i) (SI-7/8)/(SB-9/8) blends consisting of two nearly symmetric lamella-forming diblock copolymers having different chemical structures, (ii) (SI-10/53)/(SB-5/36) blends consisting of two highly asymmetric sphere-forming diblock copolymers having different chemical structures, and (iii) (SB-10/10)/(SB-9/8) blends consisting of two nearly symmetric lamella-forming diblock copolymers having different microstructures in the polybutene block. It has been found, via transmission electron microscopy (TEM), that no macrophase separation took place in each binary blend system over the entire range of blend compositions investigated. The order-disorder transition temperature of each binary blend was determined using oscillatory shear rheometry, enabling us to construct a temperature-composition phase diagram for each blend system. It has been found that the temperature-composition phase diagrams for the (SB-9/8)/(SI-7/8) and (SB-10/10)/(SB-9/8) blend systems follow nearly a Linear relationship, whereas the temperature-composition phase diagrams for the (SI-10/53)/(SB-5/36) blend system show positive deviation from linearity. The experimentally determined temperature-composition phase diagrams are found to be consistent with the predictions from random phase approximation calculations. It has been found that the microdomain structure, as determined by TEM, of each binary blend was the same as that of the constituent block copolymers.