To reveal the role of crystalline phase in the matrix polymer of gas sensing conductive polymer composite materials, a series of triblock copolymer polystyrene-b-poly(ethylene glycol)-b-polystyrene (PS-PEG-PS) was synthesized by atom transfer radical polymerization (ATRP). The copolymers have similar molecular weights but different fractions of the crystalline PEG segments. As a result, the ratio of the crystalline phase to the amorphous one can be tuned regardless of the molecular weight of the entire copolymer. The composites consisting of the copolymer and carbon black exhibit gas sensing ability as characterized by the drastic increase in the electrical resistance when meeting organic solvent vapors. It is found that matrix swelling, which provides the composites with gas sensitivity, results from the contribution of the amorphous phase. The crystalline portions prove to be unchanged in the vapors with respect to their microstructure within the time of interests. The appearance of the crystalline phase in the matrix polymer helps to push the conductive fillers into the amorphous regions, which increases the effective filler content and leads to higher responsivity to solvent vapors at lower filler loading. (c) 2005 Elsevier Ltd. All rights reserved.