Composite polymers such as high impact polystirene (HIPS) or acrylonitrile-butadiene-styrene (ABS) are widely used in manufacturing industry. In special applications, such as refrigerator liners, a good resistance to fluids aggression is required. In this study we have applied NMR and ESR techniques to investigate the effect of diffusion of fluids (light hydrocarbons) used as blowing agents for polyurethanes (thermal insulators) inside selected HIPS materials. The application of NMR relaxation analysis on materials exposed to deuterated hydrocarbons (C6D12) allowed selective observation of rubber phase modification. Both longitudinal (T-1) and transversal (T-2) relaxation provided information on polymer chain dynamics effects, determined by solvent interactions. In fact, a consistent increase in relaxation times of polybutadiene rubber phase after solvent exposition was observed. Similar results were obtained by using hexachlorobutadiene as solvent. Another confirmation was also obtained by spin-probe ESR technique. This technique could give useful insight into the molecular mobility of the two phases of HIPS. This experimental investigation provided a clear demonstration of consistent solvent penetration inside the rubbery phase of such composite polymers. (c) 2006 Wiley Periodicals, Inc.