The effect of ethylene/propylene content on the thermal degradation behaviour of ethylene-propylene-diene (EPDM) elastomers was investigated using conventional non-isothermal and isothermal thermogravimetric analysis (TGA) as well as modulated thermogravimetric analysis (MTGA (R)). Kinetic parameters of degradation were evaluated using single hearing rate data under a pseudo-first order assumption; the Flynn-Wall-Ozawa isoconversional method: isothermal methods; and MTGA (R). Although the ethylene/propylene ratio influences the onset and peak temperatures of degradation, it appears that not only the ethylene content but microstructure is also a controlling factor in determining the activation energy of degradation. Pseudo-first order analyses show that at least two different first-order degradation reaction mechanisms dominate the degradation process. MTGA (R) analyses show that the activation energy of degradation increases throughout the reaction. Isothermal TGA confirms that the degradation processes for PE, PP and EPDM are complicated and do not follow an nth order reaction path, rather they follow a random degradation process.