The spatial distribution of a midblock associating homopolymer confined within the lamellar microdomain structure of a triblock copolymer is probed with small angle neutron scattering experiments. The materials examined are poly(stryrene-b-[saturated 1,2-butadiene]-b-styrene) triblock copolymers to which a low molecular weight poly(saturated 1,2-butadiene) homopolymer has been added. The butadienes are saturated with either hydrogen, deuterium, or mixtures of the two gases in order to vary their neutron scattering contrast with respect to polystyrene. The results of contrast matching experiments demonstrate that there is a strong tendency for the homopolymer to localize at the center of the midblock microdomain. Experimental scattering profiles are modeled using one-dimensional scattering density profiles in order to obtain a quantitative description of the blend morphologies. This modeling indicates that two distinct scenarios exist for homopolymer localization in a triblock copolymer : one wherein the microdomain structure contracts and a second wherein there is an expansion of the microdomain. Possible origins of this behavior are proposed on the basis of the consideration of the configurations available to the midblock sequence, that is, tie chains that traverse the midblock domain, or loops that enter and exit the midblock microdomain through the same interface.