N-Chlorothiosulfonamides have been used to modify ethylene-propylene-diene rubber (EPDM) to enhance the compatibility of EPDM in, e.g., natural rubber (NR)/butadiene rubber (BR)/EPDM blends for ozone resistance. N-Chlorothio-N-butyl-benzenesulfonamide (CTBBS) was selected as a representative for N-chlorothiosulfonamides. In this study, we found that CTBBS behaves differently with various types of EPDM. Three types of EPDM were selected: ethylidene norbornene (ENB)-EPDM, hexadiene (HD)-EPDM, and dicyclopentadiene (DCPD)-EPDM. HD-EPDM showed the greatest effectiveness toward CTBBS-modification. However, this EPDM is not commercially available anymore. On the opposite side, DCPD-EPDM showed the lowest reactivity so that almost no modification could be realized. The result with ENB-EPDM was, that upon application of CTBBS to ENB-EPDM, gelation occurred and, therefore, a low amount of modification was achieved. With the limited modification efficiency for ENB-EPDM, there is no significant improvement when applying the modified ENB-EPDM into NR/BR/EPDM blends. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1146-1154, 2009