For the delta form of syndiotactic polystyrene, a nanoporous triclinic modification, different from the well-established and widely used nanoporous monoclinic modification, is presented. Detailed routes to triclinic or monoclinic modifications, which involve different guest removal procedures from different delta-clathrates, are described and rationalized. Surprisingly, both triclinic and monoclinic delta modifications can be obtained by both triclinic and monoclinic delta-clathrates. The triclinic modification, with respect to the monoclinic one, presents the advantage of higher uptake of pollutants having molecular volume in the range 0.13-0.22 nm(3), mainly when present only as traces in water or in air. In particular, the different molecular selectivity of triclinic and monoclinic delta modifications, favoring n-decane and n-hexane sorption, respectively, is described. This behavior is attributed to the crystalline cavities of the triclinic delta modification, which are halved in number and increased in volume with respect to those of monoclinic delta modification. (C) 2015 Elsevier Ltd. All rights reserved.