Rubber sealing materials play an important role in aluminium electrolytic capacitors, especially at high voltages and high temperatures. These materials must have outstanding temperature resistance; optimum hardness and mechanical strength for long lasting sealing results; excellent electrical insulating properties; and chemical compatibility with organic solvents and the electrolytes. Ethylene Propylene Diene Monomer (EPDM) rubber is the most commonly used sealing material in aluminium electrolytic capacitors. A series of EPDM-based rubber sealing materials have been examined and characterised by several elastomeric test methods, such as compression set test, as well as chemical compatibility and diffusion tests. The application of EPDM rubber materials in high voltage and high temperature screw terminal aluminium electrolytic capacitors has been investigated via capacitor endurance tests at 400 V and 125°C for 3000 hours. The EPDM rubber sealing materials have shown significant difference in the diffusion rate within γ-butyrolactone (BLO) based and ethylene glycol (EG) based electrolytes. The different EPDM rubbers have a noticeable effect on the capacitor’s performance (e.g. ESR). In addition, the rubber selection and ageing mechanism at high temperatures have been discussed.