A new approach to prepare and characterize double network elastomeric systems was investigated. A styrene-ethylene-co-butylene-styrene (SEBS) triblock copolymer system containing physical crosslinks was used to achieve a double network by additional crosslinking using ultra-violet (UV) light. An ethylene-propylene-diene monomer (EPDM) terpolymer system containing chemical crosslinks was used to achieve a conventional double network using UV crosslinking. Properties from conventional monotonic tensile tests, dynamic mechanical analysis, and thermomechanical properties were investigated. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical properties and lower coefficients of thermal expansion arising from a competition of the networks. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part 8: Polym Phys 48: 778-789, 2010