The separation of ethylene-propylene-diene terpolymers (EPDM) according to the three monomer units is an important task to understand the macroscopic properties of these technically important elastomers. In particular a separation with regard to the content of diene is of extreme value because the distribution of the latter along and across the molar mass axis determines the cross-linking behavior. In this study we show that high-temperature liquid chromatography (HT-HPLC) can be used for this purpose: the chromatographic retention of EPDM on porous graphite using a gradient of 1-decanol -> trichlorobenzene is a function of both the content of ethylene and diene. The contribution of the diene alone to the chromatographic retention can be quantified by calculating the difference in elution volume between the EPDM and an EP copolymer having an equivalent content of ethylene. The chromatographic separation of fully hydrogenated EPDM indicates that the additional retention due to diene is the result of its geometrical nature. Coupling the HPLC separation according to the chemical composition with size exclusion chromatography (SEC) enables to reveal for the first time the complete molecular heterogeneity, i.e. the relationship between the chemical composition distribution and the molar mass distribution of EPDM. (C) 2011 Elsevier Ltd. All rights reserved.