The influence of residual peroxide on the degradation of ethylene-propylene-diene rubber (EPDM) and detection methods for undercrosslinking were investigated. The undercrosslinking of peroxide-crosslinked EPDM could be detected by the measurement of the heat generated by the crosslinking reaction due to residual crosslinking agent with differential scanning calorimetry, not by the determination of its amount, the reaction of which was found to correlate with the amounts of remaining dicumyl peroxide (DCP) and triallyl isocyanurate. As to the DCP-crosslinked EPDM in the undercrosslinking state, softening degradation occurred more remarkably than hardening degradation, whereas the crosslinking agent remained in the rubber. By surface analysis of the aged DCP-crosstinked EPDM with Fourier transform infrared spectroscopy, we discovered that the oxidative degradation of the undercrosslinked EPDM occurred faster than in the EPDM without crosslinking agent remaining. (C) 2008 Wiley Periodicals, Inc.