Phenolic resin (PF) was incorporated into rubbers by in situ polymerization at the Vulcanization conditions Of rubbers. The PF with a localized three-dimensional network structure was formed in chloroprene rubber (CR), whereas the fabric PF was formed in ethylene-propylene rubber (EPDM). The study results showed that the PF phase was effective on reinforcing these rubbers, Depending on the morphologies of the formed PF phases, various rubber properties could be significantly enhanced. In the case of CR rubber, the tensile strength, tear strength, and modulus could be considerably enhanced, but the elongation and resilience properties were limitedly affected by PF addition. For EPDM rubber, all mechanical properties were improved, particularly the elongation, about 26% increase. The Substantial improvements of mechanical properties of CR and EPDM rubbers were attributed to their morphology, high flexibility, moderate stiffness, and excellent bonding with rubber matrix. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1177-1184, 2009