We characterize the phase behavior of a series of polystyrene-poly(2-vinylpyridine) diblock copolymers (M(W) similar to 20 000 g/mol) using dynamic mechanical spectroscopy, transmission electron microscopy, small-angle neutron scattering, and small-angle X-ray scattering. Measurements on symmetric diblocks (f(PS) = 0.5) permit us to estimate the Flory-Huggins interaction parameter (chi) for this system, which we use to place our findings on a diagram. Four ordered morphologies (lamellae, hexagonally perforated layers, bicontinuous cubic/gyroid, and hexagonally packed cylinders) are found at compositions ranging from 35 to 70 vol % polystyrene. An order-order transition separates the hexagonally perforated layer and Ia3d/gyroid phases at 38 vol % styrene. Studies of shear-oriented material indicate that the cubic microstructure does not appear to grow with a preferred orientation from the layered phase, in contrast to recent findings in other diblock systems. The occurrence of complex phases at compositions between those at which lamellar and cylindrical phases are observed appears to be a general feature of low molecular weight diblock copolymer melts.