The ordered structure was investigated in binary mixtures of an ABA-type triblock copolymer acid a homopolymer. For the study, we employed (a) binary mixtures of polystyrene-block-polybutadiene-block-polystyrene copolymer (Kraton D-1102, Shell Development Co.) (hereafter abbreviated as K1102) and homopolymer poly(alpha-methylstyrene) (P alpha MS4) having the number-average molecular weight (M(n)) of 3200 and (b) binary mixtures of K1102 and homopolymer polystyrene (PS4) having M(n) of 2800. Emphasis was placed on the effect of the chemical structure of added homopolymer on the spatial distribution within the microdomains of the mixtures. The morphology of the mixtures was investigated as a function of the volume fraction of added homopolymer (P alpha MS4 or PS4) and temperature by small-angle X-ray scattering (SAXS) and transmission electron microscopy(TEM). The SAXS profiles obtained were analyzed by using one-dimensional paracrystal analysis. It was found that added homopolymer, P alpha MS4 or PS4, was solubilized in the PS microdomains of K1102 but the spatial distribution of the added homopolymer within the PS microdomains was different. Specifically, both the results of TEM and SAXS indicate that the added P alpha MS4 was not distributed uniformly within the PS microdomains of K1102, showing a tendency of being localized at the center of the PS microdomains, whereas the added PS4 was distributed uniformly within the PS microdomains of K1102. The localization observed in the K1102/P alpha MS4 mixtures was ascribed to the : repulsive interaction between the PS chains in K1102 and added homopolymer P alpha MS chains. However, we observed a tendency that added P alpha MS was distributed uniformly within the PS microdomains of K1102 as the temperature was raised. This was explained by the temeperture dependence of the interaction parameter between PS and P alpha MS.