Nanoporous materials have been attracting considerable attention for various applications. We have reported the preparation method of nanoporous block copolymers (BCPs) by CO2 treatment. In this study, the difference between CO2-swollen and nanoporous BCPs was analyzed: nanoporous BCPs after CO2 treatment were compared with the CO2-swollen BCPs during the process in terms of nanoscopic structure and macroscopic volume. Although CO2 was depressurized well below the glass transition temperature of continuous matrix domain, both structural change and volume decrease of the swollen BCPs were observed depending on the CO2 pressure during the treatment. When the CO2 treatment was carried out at relatively low pressure (below 15 MPa), the volume decrease was well described by de swelling of the matrix domain, and internal structures were unaffected by the depressurization. At higher pressure, the volume decrease was more significant. The large decrease is most likely derived from the partial collapse of the CO2-filled space in the swollen BCPs, and is correlated with the reappearance of spatial correlation in nanoporous structures. (C) 2016 Elsevier Ltd. All rights reserved.