The phase behavior of mixtures of polyisobutylene (PIB), polyethylene (PE), and a symmetric polyethylene-block-head-to-head polypropylene copolymer (PE-PP) was studied by transmission electron microscopy (TEM) and small-angle neutron and light scattering. The thermodynamic interactions between PE/PP and PE/PIB are repulsive (Flory-Huggins parameter chi > 0 and decreases with increasing temperature), while those between PP/PIB are attractive (chi < 0 and increases with increasing temperature). When the PE-PP copolymer is added to a 50/50 PE/PIB mixture, the resulting phase diagram in temperature-copolymer composition space exhibits many of the characteristics of "fish-shaped" phase diagrams found in oil/water mixtures stabilized by balanced surfactants. This is due to the interplay between the different chi parameters that characterize the system. Lamellar phases, single droplet microemulsions, and bicontinuous microemulsions were observed. The length scales of these structures and the locations of the phase transition points on the phase diagram determined by TEM and scattering are in reasonable agreement. Phase transitions from a lamellar phase to a single droplet microemulsion phase, and from a bicontinuous microemulsion to a macrophase-separated structure, have been identified.