The mechanical properties and morphology of a series of triblock copolymer blends with midblock associating homopolymers of varing molecular weights have been characterized. The symmetric triblock copolymer studied contains polystyrene endblocks and midblocks of hydrogenated poly(1,2-butadiene) and is mixed with hydrogenated poly(1,2-butadiene) homopolymers of molecular weights both below and above that of the copolymer midblock. The rubbery plateau modulus determined by dynamic mechanical spectroscopy increases with increasing molecular weight of the homopolymer at fixed overall homopolymer content. At fixed molecular weight, the composition dependence of the plateau modulus is complex and shows unusually synergistic behavior. For high molecular weight homopolymers (i.e., with molecular weights equal to or exceeding that of the midblock), the plateau modulus increases initially upon homopolymer addition, in contrast to what is expected to occur when an additional rubbery component is added to an elastomer. Small angle neutron scattering and transmission electron microscopy are employed to determine the morphological changes induced by homopolymer addition. In all cases, the blends exhibit a lamellar microphase structure, with homopolymer macrophases apparent at high homopolymer contents. The apparent homopolymer solubility limits are found to be inversely related to the homopolymer molecular weight. Microdomain dimensions are determined by model correlation function analysis and by analysis of the interface distribution function. The results indicate that the lamellar repeat distance decreases upon addition of the lowest molecular weight homopolymer and that the microdomains swell in blends containing homopolymers with molecular weights similar to that of the midblock sequence and are unchanged for high molecular weight homopolymers with negligible solubility. The microdomain morphologies of these blends appear to be locally heterogeneous, ruling out the possibility of constructing quantitative structure-property relations. Qualitative molecular models are proposed in order to provide initial explanations for the unusual synergistic properties observed in the blends. The results suggest that the interesting mechanical response of the blends can be explained by consideration of the changes in their entanglement structure resulting from confinement of the homopolymer chains within the highly constrained environment of the midblock lamellae.