The order-disorder transition (ODT), microdomain morphology, and phase behavior in mixtures of polystyrene-block-polyisoprene (SI) diblock blended with homopolystyrene (HPS) were investigated. SI with a total molecular weight of 2.0 x 10(4) and volume fraction of polystyrene (PS) of 0.51 (designated SI-11/9) was blended with a homopolystyrene of molecular weight 6.1 x 10(3) (designated S-6). Binary mixtures of diblock copolymer and homopolymer were prepared by solvent casting. The ODT was quantitatively identified using the discontinuity observed in a plot of the reciprocal of the peak small-angle X-ray scattering (SAXS) intensity, I-m(-1), as a function of the reciprocal of the absolute temperature, 1/T, except for the mixtures showing the disordered sphere morphology for which we determined the temperature of the demicellization/micellization transition (DMT) instead of the ODT by the disappearance of the form factor peak with increasing temperature. We systematically measured the ODT or DMT temperature as a function of the volume fraction of homopolymer. SAXS data were also used to investigate the microdomain structure of the blends. Furthermore, for two blends of SI-11/9 and S-6 with volume fractions of SI of 0.77 and 0.71, we observed an order-order transition (OOT) from a cylindrical structure to a gyroid structure on heating above 110 degrees C for the 0.77 volume fraction blend and 100 degrees C for the 0.71 volume fraction blend. However, the reverse transition from gyroid to cylinder on cooling the 0.77 volume fraction blend to below 110 degrees C was not observed even after annealing at temperatures below 110 degrees C for more than 10 h, possibly due to kinetic effects. Slow cooling (2-3 h) of the blend from the disordered state led to the gyroid structure even below 110 degrees C, while the low-temperature cylindrical phase could only be accessed by fast cooling (1.5 h) from the disordered state. Experimentally determined ODTs or DMTs are compared with predictions based on mean field theory. The predicted effect of homopolymer concentration on the ODT or DMT temperature was quantitatively consistent with that found experimentally. The phase diagram of the diblock copolymer/homopolymer blend was found to show the same complexity as and similar features to phase diagrams of pure diblock copolymers.