The phase behavior of poly(ethylene-alt-propylene)-block-poly(ethylene-co-butylene)-block-polystyrene triblock copolymers with different polystyrene block lengths is investigated by dynamic mechanical analysis, transmission electron microscopy, and small-angle X-ray scattering. While the corresponding poly(ethylene-alt-propylene)-block-poly(ethylene-co-butylene) diblock copolymers show an order-disorder transition in the range of 10-20 degrees C, the attached polystyrene block prevents the two elastomer blocks from complete mixing in the same temperature range. This is also indicated by the morphologies these triblock copolymers show in comparison to the corresponding nonhydrogenated precursor polymers (poly(1,4-isoprene)-block-poly(1,2-butadiene)-block-polystyrene).