Plasma treatment of EPDM-based rocket motor insulation materials may change the peel strength between these materials and polyurethane polymers by a factor of 0.1 to 10. The matching of surface energies seems to be important for this adhesion process. The surface tension of the components was measured to between 30 and 50 mNm(-1). The total surface energy of the insulation could be increased from 27.2 mNm(-1) to ca. 70 mNm(-1) by the plasma process. Maximum peel strength could be obtained by a treatment of less than 1 min, whereas in most cases longer times gave lower values. In some cases very long treatment also gave good strength, probably due to a rougher surface structure. The rate of cure of the polymer was important for the adhesion process as lower rates of cure correlated with higher peel strengths, which can be explained by diffusion of the polymer's components into the insulation.