Microdomain structures and phase transitions of binary and ternary mixtures of polystyrene-polyisoprene-polystyrene (PS-PI-PS) triblock copolymer Vector 4111 (containing 18% PS) and the corresponding homopolymers were investigated in thin films by atomic force microscopy and in bulk by small-angle X-ray scattering. The neat copolymer shows a hexagonally packed cylinder structure. The addition of PI homopolymer caused a transition from the cylindrical to a spherical microdomain structure via a biphasic intermediate state with coexisting cylindrical and spherical domains for both thin films and bulk. In contrast, addition of a small amount (14 wt %) of minority-component PS homopolymer in a mixture leads to a significant increase of the cylindrical microdomain curvature in thin films. The influence of the added homopolymer PS on the structure of bulk samples is discussed. Addition of equal amounts of both components together does not change the cylindrical structure for bulk or thin film samples but results in an aligned cylindrical microdomain structure observed for thin films. Interdomain distances decreased upon addition of small or moderate amounts (7-38 wt %) of either or both homopolymer.