Thermal ageing is the loss of catalyst activity caused by high temperatures. The main mechanism is sintering: finely dispersed precious metal particles migrate across the surface and merge into larger particles, reducing the number of active sites. At the same time, the washcoat can lose its pore structure, in the case of alumina through phase transitions that greatly reduce the internal surface area. Both processes are irreversible and cannot be reversed by cleaning or regeneration. They are often triggered by short temperature peaks rather than normal operation. Typical situations are misfires in a gas engine, where unburned gas oxidises on the oxidation catalyst, or unexpectedly high solvent concentrations in process exhaust air. Thermal ageing shows up as a rising light-off temperature and falling conversion rates while pressure drop stays the same. Temperature monitoring upstream and downstream of the catalyst, with alarm and shutdown thresholds, helps to prevent this kind of damage.