Blends of poly(styrene)-b-poly(isoprene) (PS-PI) and poly( isoprene)-b-poly(ethylene oxide) (PI-PEO) were investigated by small-angle X-ray scattering, photon correlation spectroscopy, light microscopy, and rheology. Blends were prepared in which both diblocks had the same PI composition, f, and were investigated as a function of blend composition, Phi. We generally found a disordered one-phase region at high temperatures and two macrophase-separated, ordered phases at low temperatures. At low temperatures the only appreciable miscibility was found for f = 0.7 in the PI-PEO-rich phase. At intermediate temperatures we generally observed macrophase separation before microphase separation upon cooling. The macrophase separation boundary compared to the microphase boundary is lowered upon increasing f. For f = 0.6 and 0.7 we observe an adjustment of the d spacing of the macrophase-separated, ordered phases. The general trend of the phase behavior is described by a theory using the random phase approximation.