Order-disorder transition, microdomain structure, and phase behavior in binary mixtures of low molecular weight polystyrene-block-polyisoprene (SI diblock) copolymers were investigated. For the study, four SI diblock copolymers were synthesized, namely (i) SI-Q having a weight-average molecular weight (M-w) of 2.8 x 10(4) and polystyrene volume fraction (f) of 0.186, (ii) SI-R having M-w = 1.5 x 10(4) and f= 0.508, (iii) SI-S having M-w = 1.76 x 10(4) and f = 0.372, and (iv) SI-M having M-w = 1.55 x 10(4) and f = 0.626. Then binary mixtures of SI-Q and SI-R and binary mixtures of SI-M and SI-S, with various compositions, were prepared via solvent casting. The order-disorder transition of the binary mixtures was investigated using rheology and small-angle X-ray scattering (SAXS), respectively. Also investigated was the microdomain structure of the binary mixtures using SAXS and transmission electron microscopy (TEM). Experimentally determined phase diagrams are compared with predictions based on a self consistent mean-field theory.