The results of a calorimetric study on the melting and crystallization behavior of concentrated sulfuric acid containing dispersed single wall carbon nanotubes (SWCNT), dissolved water, and dissolved poly-p-phenyleneterphthalate (PPTA) polymer are presented. The measured reduction of the heat of crystallization is caused by the build-up of an associated layer of structured acid molecules around SWCNT and PPTA. The freezing point depression is related to the number of dissolved species such as ions. It is shown that this theory accurately describes the experimental data for dissolved water and sulfur trioxide, but not for large molecules such as SWCNT. A mechanism for the interaction between sulfuric acid and SWCNT is proposed, based on preferred adsorption of sulfur trioxide and dissociation of sulfuric acid, leading to an increased number of dissolved low-molecular-weight species. (C) 2008 Wiley Periodicals, Inc.