Butyl mastic is used in a number of industrial areas and in particular in the production of adhesives. The performance of this material can be improved by using ionizing radiation. In this work we showed that the shear forces in butyl mastic were modified when the material was exposed to radiation, and as a consequence the adhesion strength was increased. This was done by comparing the shear forces of butyl mastic before and after the exposure to ionizing radiation. The result was an increase of 440% of the average shear strength between the sealant and the substrate after the irradiation. This work can be considered as an important contribution to the field of butyl mastic. sealants and adhesives. (C) Koninklijke Brill NV, Leiden, 2012