The bulk morphologies and microphase separation behavior were studied in seven low molecular weight poly(styrene-co-isoprene) diblock copolymers with polystyrene volume fractions ranging from 0.21 to 0.76. Their order-disorder transitions (ODT) were investigated using small-angle X-ray scattering and rheological measurements as a function of the degree of polymerization (N), composition, and temperature. The resultant (chi)N versus composition plot has been compared with theories for the ODT in diblock copolymers. Near compositions of 50 vol %, our experiments agree with the fluctuation theories of both Fredrickson-Helfand and Muthukumar. At other compositions the critical (chi)N for the ODT is smaller than that predicted by the available theories. This discrepancy increases as the copolymer composition deviates further from 50 vol %. In addition, transitions from one ordered morphology to another, order-order transitions, exhibit only a slight dependence on the composition.