Poly(ethylene-alt-propylene)-block-polyethylethylene-block-polystyrene (PEP-b-PEE-b-PS) triblock copolymers were synthesized by anionic polymerization and subsequent hydrogenation of poly-1,4-isoprene-block-poly-1,2-butadiene-block-polystyrene triblock copolymers (1,4PI-b-1,2PB-b-PS). Differential scanning calorimetry and dynamic mechanical spectroscopy reveal an order-order transition which is induced by the mixing of PEP and PEE above their respective order-disorder transition (ODT).