We found that novel nanoporous structures such as ellipsoidal- or string-shaped nanopores were formed on the surface of the spherical bubbles in polycarbonate foams by annealing CO2-saturated polycarbonate. The pores were surrounded by polycarbonate nanofibrils, and birefringence with spherical symmetry was seen on the surface of the spherical bubbles. Thus, the formation of such a characteristic structure might be attributed to orientation-induced crystallization of CO2-saturated polycarbonate on the surface of the bubbles and the exclusion of CO2 from the fibrillar crystals thus obtained. (C) 2008 Wiley Periodicals, Inc.