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The performance of large screw terminal aluminium electrolytic capacitors, constructed with three different commercial aluminium cathode foils, was evaluated by capacitor endurance and internal pressurization tests; the latter using in-situ pressure transducers to monitor the pressure build-up continuously. In-situ pressure measurements showed that the pressurization process features three distinctive pressure regions: Region 1 with a steep increase in pressure; region 2 has slow pressure build up reaching a pressure off-set time and coming to region 3 when pressure increases rapidly until testing termination. The pressure off-set time varied between 200 to over 2300 hours depending on the cathode foil used. The mechanisms of internal pressurization in these three pressure regions are discussed. Capacitance measurement and SEM-EDS examination of the foils revealed that the foil with the least capacitance and morphological changes, before and after capacitor endurance test, had the longest off-set time. Presence of micro-alloying elements (Mn, Fe, Cu, etc.) detected by ICP-OES has significant effect on the performance of the foils. The internal pressurization is mainly associated with the electrochemical reactions, such as dissolution of aluminium substrate and evolution of hydrogen.