The phase state of 25 poly(ethylene oxide-b-isoprene) (PEO-PI) diblock copolymers spanning the composition range 0.05 < f(PEO) < 0.8 has been studied using small-angle X-ray scattering and theology. In addition, the thermal and thermodynamic properties have been obtained from differential scanning calorimetry and pressure-volume-temperature measurements. Twenty of the diblocks exhibit at least one order-to-order transition, and two show four ordered phases. The phase diagram consists of four equilibrium phases in the melt; lamellar (Lam), hexagonally packed cylinders (Hex), spheres packed in a body centered cubic lattice (bcc) and a bicontinuous cubic phase with the Ia (3) over bard space group symmetry known as the gyroid phase. The latter is formed for the range of compositions 0.4 < f(PEO) < 0.45 which are the highest ever reported for a stable gyroid phase. The high asymmetry in the present phase diagram is attributed to the high conformational asymmetry of the PEO and PI (epsilon = 2.72). At low temperatures, upon PEO crystallization, all phases revert to the crystalline lamellar structure (L-c). Within the composition range 0.66 < f(PEO) < 0.7 another intermediate phase is formed known as perforated layers (PL) which is clearly not an equilibrium phase. The thermal expansion coefficient was found to be a sensitive probe of the ordered microstructures.