Binary blends of four different high molecular weight poly (styrene-b-isoprene) (SI) diblock copolymers with a lower molecular weight poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymer were prepared, and their morphology was characterized by transmission electron microscopy and ultra-small-angle X-ray scattering. All the neat block copolymers have nearly symmetric composition and exhibit the lamellar morphology. The SI diblock copolymers had number-average molecular weights, (M) over bar (n), in the range 4.4 x 10(5)-1.3 x 10(6) g/mol and volume fractions of poly(styrene), Phi(PS), in the range 0.43-0.49, and the SIS triblock had a molecular weight of (M) over bar (n) similar to 6.2 x 10(4) g/mol with Phi(PS) = 0.41. The high molecular weight diblock copolymers are very strongly segregating, with interaction parameter values, chi N, in the range 470-1410. A morphological phase diagram in the parameter space of molecular weight ratio (R = M-n(diblock)/1/2M(n)(triblock)) and blend composition was constructed, with R values in the range between 14 and 43, which are higher than previously reported. The phase diagram revealed a large miscibility gap for the blends, with macrophase separation into two distinct types of microphase-separated domains for weight fractions of SI, w(SI) < 0.9, implying virtually no solubility of the much higher molecular weight diblocks in the lower molecular weight triblock. For certain blend compositions, above R similar to 30, morphological transitions from the lamellar to cylindrical and bicontinuous structures were also observed.