Blend systems of polystyrene-block-poly (ethylene-co-(ethylene-propylene))-block-polystyrene (SEEPS) triblock copolymer with three types of hydrocarbon oil of different molecular weight were prepared. The E '' curves as a function of temperature exhibited two peaks; one peak at low temperature (congruent to -50 degrees C), arising from the glass transition of the poly[ethylene-co-(ethylene-propylene)] (PEEP) phase and a high temperature peak (congruent to 100 degrees C), arising from the glass transition of the polystyrene (PS) phase. The glass transition temperature (T-g) of the PEEP phase shifted to lower temperature with increasing oil content. The shifted T-g depended on the types of oil and was lower for the low molecular weight oil. The T-g of PS phase of the present blend system, were found to be constant and independent of the oil content, when molecular weight of the oil is high. However, for the lower molecular weight oil, the T-g of the PS phase also shifted to lower temperatures. This fact indicates that the oil of high molecular weight is merely dissolved in the PS phase. The E' at (75 degrees C, at which temperature both of PEEP and PS phases are in glassy state, was found to be independent of oil content. In contrast, at 25 degrees C, at which temperature the PEEP phase is in rubbery state, the E' decreased sharply with increasing oil content. This result indicates that the hydrocarbon oil was a selective solvent in the PEEP phase. It mainly dissolved in the PEEP phase, although slightly dissolved into the PS phase as well, when molecular weight of oil is low. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121:3001-3006, 2011