We have determined the phase behavior of a series of model poly(ethylene)-poly(ethylethylene) (PE-PEE) diblock copolymers with poly(ethylene) block volume fractions (f(PE)) ranging from 0.25 to 0.46. Four ordered microstructures make contact with the ODT : spheres arranged on a body-centered cubic lattice ( space group), cylinders packed on a hexagonal lattice, a bicontinuous structure of space group , and lamellae. A fifth ordered phase, tentatively identified as hexagonally perforated layers (HPL), separates the cylindrical and lamellar morphologies at moderate or greater degrees of segregation. Binary blends of f(PE) = 0.37 and 0.46 diblocks were used to investigate the bicontinuous cubic phase region in greater detail; these experiments indicate that this phase extends as much as 100 degrees C below the ODT for (f(PE)) values in the blend from 0.385 to 0.420.